Just to be sure it's not something simple that maybe you missed, what is your aircraft path structure and which point in the path are you trying to point at?
My example structure is...
where the top folder for example is /AirCrane-compaddon
inside comp addon is /AirCrane-compaddon/AirCrane
I point to them like this
Also do you have the default hanger under "Aircraft hangers" active and you don't see those aircraft either?
Note with aircraft that are duplicates only maybe different versions how it points to their individual folders. An important point is that those folders have a folder inside them with the duplicate aircraft name or designation, ie;
mnt/IDrive/FG/Aircraft/Development Aircraft
mnt/IDrive/FG/Aircraft/Development Aircraft/AirCrane-git/AirCrane
mnt/IDrive/FG/Aircraft/Development Aircraft/AirCrane-compaddon/AirCrane
But in the launcher I am pointing at one folder back, that is important, you are always pointing at one folder back of the actual aircraft folder.
So
mnt/IDrive/FG/Aircraft/Development Aircraft
gives me every aircraft folder in Development Aircraft
but it wont give me the aircraft in
mnt/IDrive/FG/Aircraft/Development Aircraft/AirCrane-git/AirCrane
or
mnt/IDrive/FG/Aircraft/Development Aircraft/AirCrane-compaddon/AirCrane
for those I add another line to the addon aircraft, ie:
mnt/IDrive/FG/Aircraft/Development Aircraft/AirCrane-git
mnt/IDrive/FG/Aircraft/Development Aircraft/AirCrane-compaddon
those lines give me the aircraft in
mnt/IDrive/FG/Aircraft/Development Aircraft/AirCrane-git/AirCrane
mnt/IDrive/FG/Aircraft/Development Aircraft/AirCrane-compaddon/AirCrane
This gives you more details than you need, but the important part is that you do not point at the aircraft folder, you point to the folder the aircraft folder is in. The launcher searches that folder for aircraft folders inside it. If you point to the actual aircraft folder you will get a "not found" because it is looking one folder past that for the aircraft folder name.