Hello denismedia, did you make sure after adding the plugin to the addon folder to:
- add a new empty C# script, to let Godot know that this is a C# project and show the build button?
- Click the build button (little hammer icon) next to the play button?
I went ahead and tried the plugin again in 4.2.2mono and 4.3rc2mono, and in both versions I was able to install the plugin without problems. I also used the latest .NET version (6.0.424).