Plugin not showing up in menu

I don't see it having been prepared for showing in Musescore 4.4 and later

Plugins are created by users and are not official. If it doesn't work with latest version then tough luck unless the creator decides to update it. I used to do plugins but gave up because QT and other programming "things" keep changing.

Try this, I've added/changed these lines:

import QtQuick.Controls 2.15
//import QtQuick.Dialogs 1.2
...
  //4.4 title: "XenTuner"
  //4.4 categoryCode: "playback"
  Component.onCompleted: {
    if (mscoreMajorVersion >= 4 && mscoreMinorVersion <= 3) {
        title = qsTr("XenTuner")
        categoryCode = "playback"
    }
 

This does make it show in 4.4 and later as well as before

This is how it works, at least in 4.5.2. I had to modify the process to load the .json file with the settings. Simply open the .json file with any text editor, copy it to the clipboard (CTRL+A, CTRL+C), open the plugin, press 'Load tuning file', select the score (all or part of it), and press 'Apply selected tuning to score'. NOTE: To see a plugin in the menu, it must be enabled: https://musescore.org/en/handbook/4/plugins