Click on your account name in the top right and select "Account Settings"
Click the green "Add a Game Key" button on the right
Enter your code and click "Redeem Code"
Confirm the product by clicking "Continue" (optional)
*Please note! In case of getting an error ('Key is already clamed') during code activation, please make sure that the game might already be in your library. In this case, compare the purchased code and the one in your library.