Hello Marsmagnus,
Do you mean activating the dlc's so you can download them? If it is a Steam edition of the game (most likely is, however check with the retailer where you bought the game from) then you go to your Steam client, click on "+ Add game" at the bottom left-hand side of the Steam window, and then you click on "Activate product on Steam", you will be prompted to enter the Steam activation codes. Steam will then start downloading these.
The DLC's further got no extra registration on these forums.