Hi Capucine,
In my experience, conferences here in the US are pretty casual - though I couldn't say if it's more or less than in France (for what it's worth, they're about equally casual as those I've attended in Toronto and London).
I guess the norm for women is pants or skirts with nice-ish shirts or sweaters or blouses (and for men polo shirts/dress shirts without ties, sometimes jeans). There is variety though, and you certainly see plenty of people in nicer suits/ties (especially grad students presenting or on the market) and even a fair amount in jeans/shorts/t-shirts too (again, especially students). Frankly I think it might vary more by discipline than country - I've noticed geography conferences tend to be more casual and always have more attendees who are obviously unconcerned or obviously trying to make a statement (whether that be "undergraduate" or "radical" or whatever), while students tend to be a bit more professional at the sociology ones I've attended. But really all are pretty much the same spectrum of "academic casual" you'd see around any group of graduate students and faculty. I've never been to a business-school conference really, but I'd guess it will be slightly more on the professional side. In any case, as a student who is presenting, you would not be out of place if you dressed up a bit.
Specifically regarding your hair, however, you should not worry. I really don't think it matters at all - few Americans (especially academics) would think negatively or judge someone with dreadlocks; most probably wouldn't think anything at all other than that it's a personal/cultural expression. There are professors with dreads. You might not want to reinforce any lingering stereotypes by also wearing a tie-dye shirt or something, but frankly in a business-y setting it might even be a distinction from the mean that you'd be happy to have :)
Ultimately: people are laid back and nobody is thinking too much about how other people look, but there are probably more people trying to look professional (without over-doing it) than trying to look like they just rolled out of bed for a 9am class. While you don't need to be very fancy, if you're presenting you could edge on the nicer side in terms of outfits, but I definitely don't think you should worry about your hair!
