Many thanks to Mark/maeric for coming up with the solution.
The original install was not placing a Macromedia folder in the Application Support folder. So he had me manually make a Macromedia folder in MacHD > Library > Application Support AND in User > Library > Application Support. It appears it needs both. Put the MXa Registration file in there after installing (but before running) FH and it works. At last!
However, a similar, less earth-shaking problem has arisen. I can't seem to install the updater. No matter what I do, I get: "Cannot complete > File not found: Macromedia Freehand."
Another user, Claire in the UK, has had the same initial problem. But the above has not resolved it for her so far.
And FH MXa documents open in FH MX 11.0.
Best to all,
Ernie