The best place to ask questions is probably over in the original thread so the plugin creator will see.
To get the plugin to work you double click on the executable file (musescore.exe) located in the “bin” folder and allow it permission to install. To see the examples, launch MuseScore > File > Open and select one of the files.
Note that if you’re running the most current version of MuseScore (3.4.something) the developer version required for the plugin is a downgrade.
Arcandius did a very nice job on this but keeping it current with MuseScore releases is a lot of work. Shamisen players should urge the MuseScore team to include it as a standard instrument going forward.