If you did a resevation- as in put-down-5-bucks-minimum reservation, you should have gotten a reciept, and they should have taken your name, phone #, email, etc. Go back with an ID and explain, they should be able to help you out. Do it BEFORE the game releases, so you look more legit.
If you gave them no money, it's just on hold (assuming it releases like, tomorrow), or the cashier didn't know what they were doing- but no biggie. Go back and give 'em your 5 bucks, and make sure you get your reciept.
If you gave them money, but they took no info... I think you just got jacked by the cashier. Talk to the manager pronto, and hope it works out.