From the main menu, head to the Extras menu and select the SHiFT option. Sign up for an account, or log into one if you already have it, then enter the below code.

SWKTB-356W6-XC5RJ-XK6T3-HJ333

The code will work for a limited time only, and is good for use with all retail copies of the game during that period (in all regions). It unlocks three Golden Skins: Shayne & Aurox's "War and Peace" skin, Benedict's "Gold Eagle" skin, and ISIC's "I'm the Root of All Evil" skin.