Welcome to LQ. I'd suggest you go with whatever default value it suggests, but most likely it's just /dev/mouse. You can also check the existing xorg.conf or XF86Config file (in the /etc/X11 directory) to see what it's using now. (Depending on your distro, your X server config file could be either xorg.conf or XF86Config, so I'm listing them both)
To see the contents of a given file, you can just use the "more" command
Code:
more /etc/X11/xorg.conf