If a usb mouse is detected it will be used. Otherwise a PS/2 mouse will be assumed. With some multibutton mice, the extra buttons will not work unless you manually edit the /etc/X11/XF86COnfig-4 file. We hope to handle this automatically in a future release of SimplyMEPIS.
If you have a serial mouse, the 'smouse' boot argument will force the use of a serial mouse on ttyS1.