what type of mouse do you have? if you use the same mouse on a different distro, boot up that distro with this mouse, and look at that xorg.conf file and write down how it has setup the mouse, and simply use the same setup for your slack system.
i have a logitech mx700 wireless mouse and it took me days of searching and trial and error to get it to work but it finally does. heres my mouse section for my slack 10.1 xorg.conf
Option "Protocol" "ExplorerPS/2"
Option "Device" "/dev/input/mice"
Option "Buttons" "8"
Option "ZAxisMapping" "4 5"