It is -for me at least- peculiar because :
If I open in Windows Explorer the folder "F:\MyDocuments\Prepar3D v5 Add-ons\FSUIPC6", all I can see is the add-on.xml file.
If I open via the FSUIPC app in P3d Add-ons menu and click on "logging" and "open folder", it show all contents of the folder "F:\Documents\Prepar3D v5 Add-ons\FSUIPC6".
The installer points to a "Documents" folder that does not exists and creates one. However - and here is the peculiarity- it also installs a FSUIPC6 folder in the "proper" directory, containing the lone add-on.xml file.
Anyway, I now directed the installer to the proper directory and I can now see all files where they should be.
Thanks for all help.