I think this one is easy
just open your /etc/X11/XF86Config (or, if that's not your XF's config, then the one that is - but this is usually the one) and locate your mouse's config at Section "Input Device", it should look somehow like this:
Option "Protocol" "imps/2"
Option "Device" "psaux"
or whatever - these depend on _your_ own mouse config. Now add there the following line:
Option "ZAxisMapping" "4 5"
and that's it. If it doesn't work yet, then you might want to see if adding these lines help (to make sure the wheel is accepted as a wheel, not as a button - these are usually not needed):
Option "Emulate3Buttons" "off"
Option "Buttons" "3"
I guess you just need the ZAxisMapping -line. Save & restart your X, and enjoy!
to make this simple, read these too if you don't understand something (even tough I'm pretty sure you do
- I'm pretty sure you can edit XF's config while in X; just restart after editing.
- Use any text editor you wish to edit /etc/X11/XF86Config , like nano, Bluefish, vim, emacs, somekind of "notepad", whatever - that's not important. Just get the file open somehow
- I think you must be root to do this, so if you're logged in as normal user, start the editor you use from terminal and before starting, su to root. Or then just log in as root
- After saving, if you're in X, and nothing seems to happen, make sure you get your X truly restarted - either do a CTRL+ALT+BACKSPACE or then open terminal and do a "killall X" and start it again. Logout/login should do, though.