To install this, extract the Zip file contents to Blender/4.1/scripts/addons, then in Blender select the Edit > Preferences > Addons menu, search for "MDL" to find this plugin, and check the box to enable it.
@SpiderPig Since our PBR materials system is derived from glTF, this page might be useful to you:
https://docs.blender.org/manual/en/2.80/addons/io_scene_gltf2.html