Hi there, Jenny!
While you can - and it's recommended that you do - make advanced dining reservations at
T-Rex in Downtown Disney, you're really getting yourself on the priority seating list. The reservation does bump you ahead of walk-up diners, but there is no guarantee you will be seated immediately.
The priority seating is a great idea, though, if you have a general idea of when you will be there. If you don't or can't call ahead, you will just need to be prepared for a wait, contingent upon the number of priority seatings and walk-ups before you.
But you are at
Downtown Disney, so if one member of your traveling party is amendable to waiting outside with the pager, you can wander the shops and enjoy some time taking in the sights and sounds.
Remember, you can call the day before - or the day of! - and get yourself on the list.
The number for T-Rex in Downtown Disney is (407) 828-TREX (8739).
Have a great time!
See you later, celebrator,
-Deborah B.