After defeating Shub-Niggurath, you arrive back at your home base on Earth, but it's not all blue skies and butterflies. Apparently, not all QUAKE forces have been subdued. Chaos ensues as you blast your way through the QUAKE minions and toward the infested gateway. Your only choice is to find the source of evil and shut it down.
Open the Steam client, login and locate the Games tab on the top menu. Click this and then select “Activate a Product on Steam…”
Enter your unique code that's displayed on the 'Orders' page of CDKeys.com and follow the prompts to activate.
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