Take a seat and enjoy the ride - welcome to the articulated buses in Hamburg! "Three Generations" is the first add-on especially developed for OMSI 2 with not only new vehicles but a whole lot of new and unique features which as such have never before been available in OMSI 2.
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