When you create a Favorite, Photo Mechanic stores the path to that directory. If a directory is not available at that path, it's going to appear as greyed out in the Favorites panel. A directory may not be available because it is on a drive that is disconnect or because the folder deleted, or because it was a path created by a different computer and was added to Favorites via restore of Photo Mechanic's preferences from another computer.
It sounds like maybe the last option is the case?
What happens if you simply try to drag those folders to your Favorites panel from Finder or the Photo Mechanic Navigator?