I use the Content Manager and some tracks will not show up in the regular game cause of either missing files or using shader, and SOL, etc files in the track folder. So, you will find that all tracks seem to show up in Content Manager menus, but not the regular game menus. I normally install track and car mods manually even though Content Manager has another method of installs. It allows me to have better control manually and to see where they go in the Assetto Corsa folders.