1- Launch the Steam client software and log into your Steam account.
2- On the upper left corner, click on the "Games" menu and choose "Activate a Product on Steam...".
3- Alternatively, you can click on the "ADD A GAME..." menu on the bottom left corner and choose "Activate a Product on Steam...".
4- Follow the onscreen instructions to complete the process.
5- Once the game has been activated, we advise you to check the installation status. Click on "Library" and if the game's status shows "Not installed", double click on it to open the download window and follow the instructions.
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 on Steam Website