Failed to install PhotoFlow plugin in GIMP 2.10.30, Mint 20.3

I have followed the procedure described at:

I believe I have successfully built and installed the PhotoFlow GIMP Plugin; there are now no error message from the cmake, make and make install processes. The Photoflow Editor appimage is installed and executes. The plug-ins folder specified in GIMP Preferences, at:
/home/tony/.var/app/org.gimp.GIMP/config/GIMP/2.10/plug-ins
is empty. The PhotoFlow plugin does not appear in the list of Filters in GIMP.

Where is the most likely cause of my error? Is the plug-ins folder address correct?

You’re using the gimp flatpak and flatpak is a sandboxes format. You need to use an app like Flatseal to adjust the sandbox permissions to allow it execute photo flow. You also need the plugin files in the right place. Of your plugin folder is empty, likely the plugin didn’t built for flatpak.

Ah.so. ‘Right place’ is the big question: I’ve copied the 2 PhotoFlow files, which the compile process built, into 3 different ‘GIMP/2.x/plugins’ folders I have found in my system - but all in blind faith, as I don’t know where the specification of ‘right place’ is to be found. That which is revealed in GIMP Preferences/Folders/Plugins gives 3 addresses of which 2 do not exist on my system and the 3rd doesn’t create any result. Google search didn’t reveal anything I haven’t already tried.

That should be the right path.