1) In the main menu of Borderlands 2, select “Extras.”
2) Select “Shift Codes” from the “Extras” menu. SHiFT requires Xbox Live Gold.
3) From the “My Offers” screen, hit X to bring up the “Shift Code” menu.
4) Enter your 25-digit SHiFT code, then hit “Submit.”
Xbox 360 – Redeeming a SHiFT Code in Borderlands 2 : Gearbox Software Support.