To begin with
Under screen you are not supposed to put any Inputdevicescreen "Default Screen"
Inputdevice "Generic Keyboard" "CoreKeyboard"
Inputdevice "Logitech MX620" "CorePointer"
Inputdevice "Synaptics Touchpad"
Search the forum for ButtonMapping and you will most likely find the answer
I guess this post can be the one
http://www.linuxmint.com/forum/viewtopi ... ing#p92036