Hi,
I have AmRss 5.57 (I think is the latest) and downloaded the new The_Bar MUI class a couple of days ago. I just tried it now, for the 1st time since getting the new The_Bar and it works fine here.
I did have trouble initially with MUI libs, because there has been a few uploaded to Aminet in the past month or so. I advise checking that you have these installed also.