Thanks, found a link that helps:
http://www.subsim.com/radioroom/showthread.php?t=149258
CfgDate in the .mis file seems to be the issue. I notice that if I edit the .mis file via Notepad (for example), change CfgDate and save then I'm good to go; but if I subsequently edit the .mis file via Mission Editor and save it changes CfgDate back again. Guess I have to remember not to use ME after manually changing CfgDate via Notepad.
Still don't understand why other missions I see have fully-populated pulldown options of which sub class to use on that mission...