Ok, just so everyone knows, I will not go into much depth about how to get your scroll button working, but I will cover the basics:
- Open your xorg.conf file (it's likely to be hanging around /etc/X11/xorg.conf)
- Find the section related to your mouse (search for the text "mouse" or "pointer")
- Set your protocol to IMPS/2
- Add the line below the protocol: Option "Buttons" "5"
- Add the line below the buttons option: Option "zAxisMapping" "4 5"
- Save the xorg.conf
- Restart the X windowing system. If you did you editing from within X (using KDE, GNOME, or other environment) you should be able to hit CTRL-ALT-Backspace to restart X.
These instructions will likely vary depending on your distribution. Some distributions (like Fedora, SuSE, Ubuntu, etc) don't seem to play well with manually-edited files and prefer that you use their graphical interfaces to change settings like this. But you should also not have issues using the scroll button on your mouse with the distributions listed above.