Ok so working at gamestop I figured that the whole having to reserve it thing was a load of BS and I was right (I think). I entered totally bogus information and still got the demo. There is a 1 in a 1000 chance that everything I entered in just happen to be a random reserve for this game, but I seriously doubt it.
Step 1:
http://www.gamestop.com/gs/landing/l4dearly/
Step 2: Write in the information.
At the top right click of that address look for a link that says Store Locator. Write in your zip code. After each store location there should be a # and then anywhere up to 5 letters. Copy that as it is needed for the Transaction ID part.
For the "transaction id" write [
The Store number you just looked up (mind you if it's only 3 numbers you have to put enough 0s in front of the store number to make it 5 numbers... if you're confused see the example they wrote)
]/01-[any number 001-030]
ex. 00372/01-014
The 01 is the register (usually 2 per store) and the last numbers after the - is the transaction number. I said do it up to 30 because some stores don't even get 30 transactions a day so why risk it.
Then write in your email address (keep it real cuz they will email the code to you) and your name.
Step 3: Tell me if this works. Like I said it worked for me, but there is a possibility that I completely randomly guessed a transaction where someone at my store actually got a reserve for the game.