For those of you who may be having issues using a USB Mini-Pac with your MAME cabinet, make sure that you do not have PS2 peripherials plugged in. Use USB only as PS2 seems to interfere with the USB Mini-PAC (i.e. trackball not working, mame recognizing PS2 mouse instead of trackball, etc).
I spent about two weeks frustrated because I couldn't figure this out. Hopefully this post prevents someone else from wasting the same time and energy on setting up their MiniPac.