There are no real up-to-date versions of FX Foundry or MM. Use the latest ones, FX Foundry is from 2008.
MM 1.3.5 works perfectly in 2.8 64-bit, even it´s 32-bit. Gimp 2.8 supports old 32-bit plugins, if You didn´t untick that option upon installing.
FX Foundry download link:
http://downloads.sourceforge.net/gimpfx ... -2.6-1.zipI´ve picked some favorite scripts from it, can´t verify they all work without errors.
It´s advisable to keep the FX Foundry in it´s own folder and add it from preferences.
MathMap:
https://sourceforge.net/projects/gimp-packagers/files/Installation instructions are included. Place the mathmap folder in .gimp folder, You can drop the files in plug-ins folder
to .gimp/plug-ins if You wish or rename it to MM for example and add the folder via preferences.
I do that always with plugins that have .dll´s to prevent conflicts.
edit: Check the pinned topic about MM extra expressions, You might find 2 or 3 new ones from there.