Ensure that you are activating your game through the Steam application. The Steam website cannot be used to activate a game.
- Download and install the Steam client on your PC.
(https://store.steampowered.com/about/), then sign up and log in.
- Locate the Steam key in your confirmation email, and copy it to your clipboard by pressing “Ctrl+C”.
- Open the Steam client and locate the “+ ADD A GAME…” button on the bottom left of the window. Click this and choose the “Activate a Product on Steam…” option.
- Paste the key from your clipboard in the “Product Code” section when asked, and follow the rest of the prompts to add the game to your library.
- Your game is now viewable in the “Library” tab, and is available to download/install when you’re ready!
You can find additional support for the Steam client here: https://help.steampowered.com/en/wizard/HelpWithSteam