1. Open your download page from the email you received from cdkeys.com and locate your GOG key.
2. In another window, log into your GOG account. Navigate to your library and scroll to the bottom of the page. There, click on the "REDEEM CODE" link.
or you can go to this link directly: http://www.gog.com/redeem
3. On the next page, paste your code into the indicated field, and click the green "Continue" button.
4. On the next page, you will confirm the order and make sure you are redeeming the correct game. Click the green "Continue" button to move to the final step.
5. On this page, you will "pay" for your order. As you have already purchased your key(s) from CDkeys.com this field will then show up as "Free". Don't worry, no more purchases are necessary. Click the green "PAY FOR YOUR ORDER NOW" button to finish your key redemption.
6. And that's it! From here, you may navigate to your GOG library where you will see the new title waiting for you. Now go forth & game on :)