ok so you do have the EDDF ground net... that's good...
do you have ANY additional scenery directories defined in the launcher?
i ask this because when FG searches for scenery directories, when it finds the one it is looking for, it stops searching for it further down the list... so if you have a scenery directory that contains Airports/E/D/D with EDDF files in it, it will stop there... if the groundnet file is not in that directory, FG will not have one to work with even if there is another directory further down your scenery directory list with the groundnet in it...