Update: We understand that some of you were having issues with the DLC being in polish. This issue should now be resolved. Thanks for your patience and understanding.
Original: The way to redeem your Dying Light DLC is a little different, so we thought we would give you a heads up.
To redeem your codes, you simply head to https://boxoffstore.com/en/coupons/redeem/, sign in via facebook or sign up for an account and redeem your code. You'll then find your code in the collection selection section and from there you will be able to download it.
Enjoy your game.