Follow these simple steps to activate your new game on GOG.com.
Step 1: Head over to GOG.com. Sign in or create a new account.
Step 2: Once logged in, select "redeem code" from your profile menu or simply visit the following link gog.com/redeem.
Step 3: Enter the cd key that you would like to activate on GOG and press "continue".
You can also redeem your GOG.com code using GOG Galax