see mouse(4) manpage
Strangely, XFree86 requires the mousewheel buttons to be the last who on the mouse. Some mice, like the 4 button Logitech Cordless Desktop Pro mouse, uses buttons 4 and 5 for the mousewheel. This means that if you use "ZAxisMapping" "4 5", what is the correct setup, the last button will be unusable. Instead, use a normal configuration. An example:
Option "Device" "/dev/mouse"
Option "Protocol" "MouseManPlusPS/2"
Option "Emulate3Buttons" "no"
Option "ZAxisMapping" "5 6"
Option "Buttons" "6"
(This example configuration is for a 4 (6) button Logitech Cordless Desktop Pro Mouse)
This makes all the buttons usable, but the mousewheel is mapped all wrong. Scrolling down gives up, and the last (side) button functions as the down part of the mousewheel.
To solve this, use xmodmap -e "pointer = values" to remap the buttons in of your desktop init scripts (usually placed in /etc/X11/xdm/Xsession)
You can also use /opt/kde/startkde or
/usr/bin/startkde if you are using KDE,
/usr/bin/gnome-session if you use Gnome.
To find out where your startkde, Xsession or whatever is, type 'locate startkde'.
A valid xmodmap command:
xmodmap -e "pointer = 1 6 3 2 4 5"
This remaps the mousewheel correct, and also remaps button three to button six. If this does not work with your mouse, then read the xmodmap manual page and experiment. You might even find the right values by reading your mouses manual, if it came with such a thing.
This part of the link I provided didn't help? Take a look at the man page for xmodmap, from a command line type man xmodmap and read the man page.