"After reading several articles and a book about Buddhism, I now feel a bit uncertain about what Buddhism actually is about."
Buddhism, in essence, is about the four noble truths -- every single "Buddhism". It's about suffering, happiness, eliminating the former and cultivating the later. (See also Buddhism is kind of depressing).
"Is Buddhism about finding happiness in Nirvana? Is that really it? Is it not about being happy where we currently are?"
By the early textual tradition, in it's most dedicated form (e.g. monastic), Buddhism is the training to attain Nirvana, the permanent end of suffering. The Buddha said:
Bhikkhus, if wanderers of other sects ask you:
"For what purpose, friends, is the holy life lived under the ascetic Gotama?"
— being asked thus, you should answer them thus:
"It is, friends, for the abandoning of the fetters ... for the uprooting of the underlying tendencies ... for the full understanding of the course … for the destruction of the taints ... for the realization of the fruit of true knowledge and liberation ... for the sake of knowledge and vision ... for the sake of final Nibbāna without clinging that the holy life is lived under the Blessed One."
-- Annatitthiyapeyyala Vagga
If all Buddhism was about was being content about our current state in life, and being happy about whatever we do and what happens to us, then there wouldn't be much to teach -- or to learn.
There are buddhist teachings that focus on our contentment with whatever situation we find ourselves in, of acceptance, but there's a particular function, attitude and scope to that exercise. For example, this "acceptance" does not extends to passively accepting hindrances like sloth, to accept our lack of striving, lack of purpose or lack of dedication to eradicate defilements of the mind.
Unfortunately, this "just be happy mantra" is so repeated in buddhist circles that it seems to be equated with the entirety of Buddhism, leading to a lot of misunderstandings.
"Am I not allowed to dance? To sing? To fall in love?"
The degree to which Nirvana is pursued in this very life depends on the particular buddhist school and the personal inclination of the buddhist.
Again, by the early textual tradition, the Buddha was very accommodating to listeners of different inclinations. Some of his disciples lived "normal" lives, married with children, jobs and probably entertainments of many sorts. In buddhist lingo, these are called laymen/laywomen or householders.
However, "normal" life is a huge obstacle for attaining Nirvana. Being sensitive to the fact that many refuse to renounce it, the Buddha did not appear teaching them the more austere practices of renunciation that are appropriate for monastics fully dedicated to the path. Particularly, if these listeners have no inclination to renounce, e.g., singing and falling in love. Instead, he taught them what they could do: abandon evil actions, be good, be virtuous. And if the listener is so inclined, he seems to have also taught deeper practices like meditation as well.
So, Buddhism is a gradual path of happiness, it's not 'Nirvana or nothing'. There is progress and there are lower attainments possible even for a lay person.
At the very least, by training on the most accessible teachings alone, one may be able to create a more peaceful, easier and happier life now and for the rest of one's life. And finally, according to the tradition, one is more likely to be reborn in a realm with less suffering (until, that is, one dies from there and reappears somewhere else, a better or worse realm).
Thus, the saying in the buddhist texts:
The dhamma taught by the Buddha is good in the beginning, good in the middle, good in the end.
The teachings promotes happiness now, happiness later, and happiness after that.
That's a gist of the Buddha's teaching: extinguish suffering permanently, or at least lessen suffering.
"Is the purpose of Buddhism to become a munk or nun?"
Being a monk or nun is, theoretically, the most adequate and, perhaps, safest route towards Nirvana. "Theoretically", because monasticism around the world have difficulties living up to this ideal.
However, lay life is not disapproved or frowned upon at all. Moreover, lay buddhists are at the core of the support of monastic life and comprise the two parts of the fourfold sangha: laymen and laywomen.
So, really, it's a matter of personal choice.
"Is it "bad" to want to be happy and/or content?"
This is the whole point of Buddhism. However, there are different kinds of "happiness" and "contentment". Some of them comes back and bite us back in a pernicious way and thus are sources of suffering. For example, successfully stealing money can be a source of happiness but isn't it also a source of suffering?
Thus, abstaining from stealing is a Buddha's teaching and is not something so incompatible with lay life, certainly a lay buddhist can practice that without inconvenience.
But as one looks for a more stable kind of happiness, one discover other things that also have drawbacks. These, in Buddhism, are called sensual pleasures: those born from the senses and contact (e.g. the sight of an enticing form, the hearing of an enticing sound, etc).
At this point, the Buddha usually addresses the monks and nuns only, and speaks quite directly about the essential nature of sensual pleasures:
"Again, with sensual pleasures as the cause, sensual pleasures as the source, sensual pleasures as the basis, the cause being simply sensual pleasures, people indulge in misconduct of body, speech, and mind."
-- MN 13 (Bodhi trans.)
In a more detailed and "technical" buddhist jargon, the Buddha says:
“When one abides inflamed by lust, fettered, infatuated, contemplating gratification, then the five aggregates affected by clinging are built up for oneself in the future; and one’s craving—which brings renewal of being, is accompanied by delight and lust, and delights in this and that—increases. One’s bodily and mental troubles increase, one’s bodily and mental torments increase, one’s bodily and mental fevers increase, and one experiences bodily and mental suffering.
-- MN 149
Then, in higher training, the Buddha teaches sense restraint -- which is not something he is seen teaching lay followers as far as I can tell.
But contrary to popular thinking, the point is not to create a depressed monk or depressed nun. These sensual pleasures are "substituted" by other kinds of happiness that the monastics trains and cultivates, that are less problematic and lead straight to Nirvana.
To name just a few, the happiness of renunciation (e.g. perception that monastic life is simpler and worldly life and commitments as a burden), the happiness of full dedication to dhamma (to practice, learn and teach the eradication of suffering), the happiness of jhāna (deep meditation states).
Because these lead to less desires, it's also easier to be content, to experience deeper contentment and long lasting contentment. Finally, when superior and simpler kinds of happiness are cultivated and grow, people tend to perceive sensual pleasures as inferior, gross, a burden, even suffering.