Open Explorer and go to Tools -> Folder Options -> File Types and find "MXM" in the list. Click "Advanced". You should see an action called something like "Open with Material Editor". Make sure it's the default, select it and click "Edit". In the "Application used to perform this action" make sure you have the following string:
Code: Select all"C:\Program Files\Next Limit\Maxwell\mxed.exe" -mxm:"%1"
Of course, replace the path above with the path where you have 1.5 installed. Don't forget the quotes around %1 and don't put any spaces before or after the colon.
Anyway, this is not a Max plug-in issue. :)