Tyr is right. You have to play 500 raked hands before you can cash out. If you play at the lowest limits, I would say that you need to play app. 700 hands in order to reach the 500 raked hands. This is quite easy to do. And the worst thing that can happen is that you'll lose their money. But chances are that you'll be able to at least break even and cashout over 100$.
I actually cashed out something like 225$ after my 500 hands. Not too bad for free money. I then did some of the other promos with that money I and I now have some nice poker chips and a poker table for my home games.