If that Configuration Doesn't Work, try this one:
Section "InputDevice"
Identifier "Mouse1"
Driver "mouse"
Option "Protocol" "ExplorerPS/2"
Option "SendCoreEvents" "true" # Not sure if this line is needed ??
Option "Device" "/dev/input/mice"
Option "Buttons" "4 5" # side buttons scroll up/down
Option "ZAxisMapping" "6 7" # wheel moves forward/back in the browser
EndSection
Interchange the "4 5" with the "6 7"
FYI: This was the only way I could manage to get xev to even recognize the 2 side buttons on the mice at the same time that it recognized the wheel movements. Notice that the buttons now scroll the mouse, and the wheel moves forward/back (next/back) in a browser. To fix this I had to remap the mouse buttons by adding the following line to my startup script. (read below on which file this is for you)
xmodmap -e "pointer = 1 2 3 6 7 4 5"
Source:
http://gentoo-wiki.com/HOWTO_Mouse_Nav_Buttons
Jake