Commandos 3: Destination Berlin is a game of real-time tactics that places you in command of an elite unit of Special Forces behind enemy lines in the European theater of World War II. From the shores of France to the heart of the Third Reich, strike fast from land or sea infiltrating hostile territories and conducting raids to disrupt...
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