The Sequel to the award winning X²: The Threat introduces a new 3D engine as well as a new story, new ships and a new gameplay to greatly increase the variety in the X-universe. The economy of X³: Reunion is more complex than anything seen in the X-universe before.
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