While you're there, take the opportunity to backup your saves, craft files, etc. Once you know where it is, you can go direct to it, no need to go via the Steam client app. That's just to help you find the location. You'll know if you're in the right place by the presence of the GameData directory. That should open a window on the main KSP install directory (buried a few levels deep inside Steam's data directory by default). Right-click on KSP in your library, and select properties from the menu.
The Steam client should have a 'browse local files' option in the properties window for KSP.
