I can think of a few reasons...
1) if you're under age 13, it won't let you play...and since you're not...
2) you entered your birthday the European way (dd/mm/yyyy) instead of the American way (mm/dd/yyyy)
3) it's not available in your country (if you're in the US, this won't apply)
4) there's a block on either the site, or the computer you're using preventing you from playing.
Those are the only ones I can think of right off the top of my head. If you're doing everything right, and it still won't let you play, I think there's a way to contact the site to reset it, but I've only heard that from a very few people, and they might know something the rest of us don't.
Anyone else have any ideas?