The Plus! menu wasn't showing up, so I had to re-install Plus!
Now that it is showing up (I haven't rebooted yet), I still can't find an entry about "software compatibility."
Edit: Okay, someone directed me to this thread:
http://shoutbox.menthix.net/showthread.php?tid=81749
and I now realize that Messenger isn't picking up on the error. The Plus! menu does
not display those options - it looks normal.
However, I still have the problem of Plus! not loading with Messenger whenever I restart my computer. If I un-install and re-install, Plus! works fine until I reboot/shut down.
Are there any more things I can try?