I used 'regedit' to find and then remove all instances of
disk management for WHS . saved my work then tried to reinstall correctly, ie. the add-ins window. Got the same message, as above. Repeated the steps above, w/ same result.
So to heck w/ it I went to sleep, tried to reinstall next day, same error. 2 days later,
disk management for WHS is installed and works great.
Ain't microsoft software miraculous? Essentially, the problem solved itself.
Thanks for the advice Ken. Whenever, I post here, I hope you will respond, always good advice. Jim