To redeem your item in-game, seek out Landro Longshot by the docks of Booty Bay in Stranglethorn Vale. When you find Landro, he'll ask you which item your code is for; check your loot card if you're not sure which item that is. Select the item from Landro's list of available promotions, and then enter the in-game code you wrote down.
The card doesn't say what it's for and anything I try gives the error "couldn't validate code, please try again..."
Edit: I got it, found the gift box.