Shortly after landing on an alien surface, you learn that hundreds of your men have been reduced to just a few. Now you must fight your way through heavily fortified military installations, lower the city's defenses and shut down the enemy's war machine. Only then will the fate of humanity be known.
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