I'm not with Laminar Research, just a fellow simmer from the UK and I have come across the type of problem before.
To prove a point, I have just downloaded Ground Handling Deluxe (GHD) directly from JARDesign's website and found again that there are two versions of the software, one for Windows and the other for MACOSX. You therefore have to be careful which version you actually install into your plugins folder!
The first question therefore, is which operating system are you running?
Secondly, which version of GHD have you installed? If you're not sure about this, go into File Explorer and open up one of the folders in GHD, say the C:\X-Plane 11\Resources\plugins\GndHandling\Cars\Ambul folder.
If you see the characters ._ (dot underscore) in front of the file names, then you have the MACOSX version installed. If there are no ._ (dot underscores) in front of the file names, then you have the Windows version installed.
Perhaps you could have a look into this and advise further?