Checklist:
You used the correct installer for Gimp 2.8 https://gmic.eu/files/windows/gmic_gimp2.8_qt_win64.exe
Installed to C:\Users\yourname.gimp-2.8\plug-ins as a complete folder (check it is there)
Made a path to that location in Edit → Preferences → Folders → Plug-ins (check that and check again)
Restart Gimp.
If the problem is Gimp 2.10 help, then there is a very up-to-date (25 June 2019) PDF version at http://gimp.linux.it/www/meta/ (English or Italian) 1000 pages indexed, searchable about 43 MB