Musicmatch Jukebox does not use the ID3v2.3 POPM field (popularimeter), which is used for rating in MC12 and many other programs.
However, MC12 supports the proprietary "Musicmatch Preference" tag. You can make it visible by adding the Preference column to a view (Use the details view mode).
You can transfer Musicmatch Preference values to MC's star ratings by creating six playlists, one for each Musicmatch rating. Sort your view by the Preference column and add the files to corresponding playlists. After that you can easily select all files in a playlist and rate them all at once.
Musicmatch has six default preference values and the star rating system has only five stars. A workaround is to use the "no stars" value for the lowest Musicmatch value. Alternatively you can combine two of the values.
This thread has more info about the supported Musicmatch tags:
http://yabb.jriver.com/interact/index.php?topic=33491.0