Activating CD key from your physical copy of the game on Steam Game activation
  1. Launch the Steam client software (or download it first at and log into your Steam account.
  2. Click the Games Menu.
  3. Choose Activate a Product on Steam.
  4. Enter your CD key and click Next.
  5. Follow the onscreen instructions to activate a product and complete the process.

Please note that you can only activate your game on an account where the game has not been activated yet.