Hi, all. Probably found a solution. If you installed Notepad ++ in the default directory (C:\ProgramFiles\Notepad++), then with a high probability you will not be able to install the editor plug-in. And now I will explain why this is so and show how to solve the problem.
Firstly, for installation in the ProgramFiles, elevated privileges are required,
Steam started with the rights of a normal user (by default) and does not have elevated privileges and requests them if necessary (you've all seen these windows when installing games or programs in Steam). Because usually after installing the game or the any program through Steam is usually not required administrator rights - this error is quite unusual - because we need to install a plug-in from the program running under the normal user in the folder that requires elevated privileges.
Now that we have understood the cause of the problem - we have almost found a solution
Close Steam (and Notepad++) and run it as an administrator:
Run Spintires: Mudrunner Editor and install Plugin as usual (Settings menu -> Path -> press Install Notepad++ plugins, and select where Notepad++.exe located)
After that open any truck\level file and check plugin status:
As you see it works fine
Hope this help you and sorry for my english;)