Is ALT + T the "Extended" list?
Produces nothing:
%POPULARIMETER% instead of %rating mm% in your screenshot #3
I followed the instructions and previously read the linked page a few times before I posted my question, and is the only result that also shows when I press ALT + T (For example purposes, I intentionally marked all other fields [keep] on "Tag" and "Panel" lists). Note: used brackets surrounding "keep" because the angle brackets don't show up in this post:
Load only 1 song into Mp3tag you can see the content in POPULARIMETER in the Extended Tags window.