OK, I think I have this working. First in MB go to Edit/Preferences/General, check the box for "enable MusicBee in Windows Explorer and context menus".
Then in file associations use :
"C:\Program Files\MusicBee\MusicBee.exe" "%1" /QueueNext
or
"C:\Program Files\MusicBee\MusicBee.exe" "%1" /QueueLast
depending on where you want the double clicked song to be queued.