OK, curiousity killed the cat, I downloaded a new VST plugin never used before, and placed the dll in my standard folder for VSTs that my MusicBee vst host plugin points to in its path statement. And, the new VST plugin DOES show up just fine in my list of selectable "DSP Plugins." And if selected, works.
Anyway, this disproves the theory that MusicBee is no longer scanning the VST folder after initial installation of the vst host.
jack19277, I think you may be dealing with a Windows permissions issue. I run Windows 7 Pro SP1 as an Administrator (carefully), MusicBee Portable installed to C:\MusicBee, and my VST plugins folder is at C:\VSTPlugins, so both outside of the normal Windows hierarchy. Why don't you try creating a new VST folder right off the drive root and not in Programs, and pointing MB to that instead. Any luck?
"For completeness sake", I'll mention that while unchecking the new plugin in MB and then deleting it from the VST folder does remove it from the list of selectable DSPs, it does not delete its entry in <PlayerDspState> within the MusicBee3Settings.ini file. Neat-nicks like me can clean it out manually however