X11: Have wheel on mouse must use in x
(last edit: 2001-05-01)
                   <http://www.freebsd.org/FAQ/x.html#X-AND-WHEEL>

Change your the pointer device in your /etc/XF86Config file to:

Section "Pointer"
   Protocol        "Logitech"
   Device          "/dev/psm0"
   ZAxisMapping    4 5
   Buttons         5
EndSection 


Yes you have to specify 5 buttons even if you don't have 5 buttons. 
This is what needs to be done:

You'll have to translate mouse button 4 and mouse button 5 events into key events. Thus, 
you have to get the mouse driver to translate mouse wheel events to button 4 and 5 events.