What confuses me is (and I'll ask when I trade it in unless somebody else already knows) that at GS you get $5 towards a reservation of GoW3 w/purchase of Bulletstorm. So technically you should have two beta keys anyway...
Your Bulletstorm disc doesn't require a key, it is a menu choice, so you have to have the disc. it should be active on April 18. I don't know if you have to have the EA online pass. (It is a multiplayer beta, but the publisher for Gears is Microsoft.)
You can go to GameStop and show your receipt. They will print off a key that will be good on April 25.
You do get one week early access to the beta for Bulletstorm Epic Edition.