Put the code into Steam
Right click in the Impulse on the SM - "Show First Run Info". There is your Elite pack code.
GameStop printed a code on my receipt, I typed it into Steam and it downloaded my Armor sets
I bought a boxed edition: as far as I can tell, there is no code on my receipt...
The employee probably forgot to give it to you. I'd go to the store and bring your receipt, they'll be able to print one out for you.
I got mine shipped to me... ...I can't just go to a store cause I'm on an Island in the Puget Sound. I think I'll end up emailing Gamestop and THQ. There's also a chance that my mother, who ordered the game, received and email and just didn't forward it to me.
Look on the back cover of the manual