It would be helpful if you were a little more specific, like about what "display" you are asking about.
So, assuming you are asking about the timeline at the top of the arranger, you must click on the counter in the transport controls and then click on "Display milliseconds".
I have been through the initialisation file for MMM looking at the parameters which are set there and there is no setting to make the display in milliseconds.
@smartsmurf
. . . . Since bars is the way to measure the length of music.... . . . .
Very true, however if you are producing music for, for example, a video, commercial etc then you are limited by time constraints.