Activating a Product on Steam
Please follow these instructions to activate a new retail purchase on Steam: Launch the Steam client software and log into your Steam account. Click the Games Menu. Choose Activate a Product on Steam... Follow the onscreen instructions to complete the process. Please see the article for more info on registering keys.
Ensure that you are activating your game through the Steam application. The Steam website cannot be used to activate a game. If you have not yet done so, you can download and install the Steam application by clicking on the green "Install Steam Now" button here: