Video Instructions for Windows OS:
Video Instructions for Mac OS:
1) Go to https://store.steampowered.com/about/
2) Click on "Install Steam" to download the application.
3) Once downloaded, install the client.
4) Once installed, find and open the Steam application on your computer.
5) Create a Steam account and login.
6) Once logged in to your client, click on “Add a Game” at the bottom left of the application window and click on “Activate a product on Steam”
7) Enter the game key provided and download the game.
You can only redeem the key via the downloaded and installed Steam application and not directly on their website.