First and foremost: The game HAS to be installed in the default Steam directory. A bug (probably related with Steam itself) prevents workshop items from showing up if the game is installed elsewhere.

Second: You subscribe to the workshop item on Steam and it should show up in the custom ship/track sections in the game.

Third: Steam needs to be activated (read online or not inhibited by some other program) otherwise Steam's DRM will just lock the workshop items out, if you haven't moved them into the game's folder manually.