The MUI opened. I just finished to post-upgrade upgrade (firmware).
I opened OneNote 2013 for the first time since upgrade and got the "Do you want to make this version the default?" dialog. I clicked yes, but MUI version still opens with the pen click.
Having it work "you way" (as the Burger King says) would be best, but I'm just so delighted it finally works at all for me. I ,mostly will use it at work while standing and using OneNote as a white board to draw and wrote with pen, the MUI interface is fine. When at home with mouse and monitor I'll still use desktop OneNote 2013.