Well, that depends on which version of MMM you are using. (You did bother to tell us.)

Normally, you would use the karaoke function.  This may or may not work depending on the nature of the vocals.  It seems to have trouble removing really deep voices, and noise, such as sibilence (Sssssss....), tends to leave artifacts behind.

It appears that the karaoke feature does not work at all in MM-15.
You may have to find a plug-in elsewhere to do the job.