SHiFT is is a program set up by Gearbox Software to reward fans with additional in-game items for Borderlands 2, such as Golden Keys, loot and other goodies -- it has nothing to do with redeeming pre-order bonuses like Premiere Club.
Please note, you must be 18 years or older to sign up for SHiFT.
If you link your SHIFT account to any given platform (PSN, Steam Xbox 360) from within Borderlands 2 -- not the site itself -- you'll receive a bonus Golden Key!
Upon linking your SHiFT account from in-game (Main Menu -> Extras -> Shift Code) the bonus Golden Key will be automatically added to your inventory, not sent as a code in a separate e-mail.
If you signed up for SHiFT and need additional assistance, including issues with signing up with the incorrect email address, issues confirming your account, or if you did not receive a Golden Key, please submit a ticket directly to Gearbox Support here:
Gearbox is looking into variety of ways for giving away SHiFT codes. keep an eye on the forums (forums.gearboxsoftware.com), Twitter (twitter.com/gearboxsoftware) and Facebook (Facebook.com/gearboxsoftware) as well as GearboxSoftware.com for updates on that front.
You can also find more information in the Gearbox Knowledge Base.