1 .Install the Steam Client Application. You can download it here.
2 .Launch the app and sign in with your Steam account.
3 .Select "ADD A GAME" as shown in the picture below, then choose "Activate a Product on Steam".
4 .In the Product Activation window, select "Next".
5 .Agree to the Steam Subscriber Agreement by selecting "I Agree".
6 .Copy your Game Key and paste it into the Product Code field, then click "Next".