That's common and by winning the FR$ first you kinda screwed yourself. All sites, that I am aware of, have requirements before being able to withdraw FR$. # of hands played at a certain level or amount of rake.
In that way, the site gets to essentially keep a portion of the "free" money. You need to risk it against other players and they get the rake from all those hands.
These restrictions usually do not apply to players that have made a deposit first, but again, that is site specific.
I looked through FullTilts' site and couldn't find the specifics in the FAQ guide so you'll probably need to contact their support for the information.
FYI, just making a deposit now, will not clear it up. The restriction still applies to the winnings, so don't deposit $50 expecting to withdraw $450 the next day.