OK, PROBLEM SOLVED! Rather than fight with Sonar, I:
1) Created a VSTplugins folder under Cakewalk in the Programs folder.
2) Moved the Session Drummer 2 folder & all its contents from my default vstplugins folder (where I think I may have manually moved it long ago, probably having something to do with my problems).
3) Using the Cakewalk Plug-in Manager, added the new VST folder I created within the Cakewalk folder.
4) Rescanned all VST's.
5) STILL DIDN'T WORK!
6) So I took Dan's cue from above, used Regedit to change the contents path to my newly created folder, AND...
7) All works perfectly!
Interestingly, Session Drummer 2 has disappeared as an available plug-in, which is fine with me, even though its .dll file is still in the Session Drummer 2 folder.
Also, when I "F5" refresh my program list from within Session Drummer 3 the Hip Hop expansion pack is now in the top level hierarchy of choices. Cakewalk's internal tools placed all the expansion pack programs right in front of my face even though they are in a different directory from the Session Drummer 3 folder. Thanks, Cakewalk!
Thanks to you for all the assistance!
Richard