I assume that the file format is MP3 (MC uses ID3 tags only with MP3 files).
By default MC12 updates the POPM ID3v2.3 field in MP3 files when ratings are changed. I have tested this and it works. Have you changed any tag related options?
Also, have you checked the format info? (Click the "Format" field inside Action Window > Tag for making it to show a full tag dump. This shows the tag fields directly from the selected file.)
Is the following line visible:
POPM: <unknown data type> ?
You can also open an MP3 file with a text editor like Notepad (this is usually very slow) or with a hex editor (this is faster) for checking if the POPM field is present. All ID3v2.3 tags are in the beginning of the file.
However, as you said, it seems that Tag&Rename can't display the POPM tags that are created with MC12, even though e.g. Window Media Player 11 or
Mp3tag can do the same without problems. The used tagging format must be slightly different, but I can't say what exactly is the difference.