I was not; I'm pretty new to running XP under MacOS, having retired my PC last year, and was not aware of that option. I went back and checked the box to start XP with Rosetta. Launched XP12, selected the MU-2 and, this time, got the activation box. After filling it out and submitting, nothing happened; the MU-2 still doesn't appear in my licensed list. I did verify that I was running under Rosetta.
Maybe I need to un-retire my gaming PC?