how do i redeem code

Xbox: Go to the marketplace and redeem the code like usual. Xbox Guide Button > Left > Left > Scroll down for Redeem Code.

PC: Your code is automatically applied. If you wish to check the codes applied, then go to your library, right click on the game, and then push View Downloadable Content.

PS3: I don't have one, so you are on your own.

Not if you bought your game at a brick/mortar for the PC they aren't... got to the Steam menu bar and select "Games">"Activate a product on Steam", and enter your code there.