Look in your /etc/X11/XF86Config file.
You should be looking under InputDevice, where Identifier is Mouse ( or Mouse# )
You should have an option in there that looks like this:
Option "ZAxisMapping" "4 5"
Be forewarned, sometimes a desktop can be cranky...for the most part I can scroll up and down no problem, but every now and again ( usually only in Mozilla ), I can scroll down, but as soon as I go all the way down, I can't scroll up anymore ... oh well.