Please follow these instructions to activate a new retail purchase on Steam:
- Download and install Steam client from here: https://store.steampowered.com/about/
- Launch the Steam client software and log into your Steam account.
- Click the button in the bottom left corner that says "+ Add a Game…"
- Choose "Activate a Product on Steam..."
- Follow the onscreen instructions to complete the process.
- After successful activation, your game will be visible in your Library.