I've read a few related questions:
Enable horizontal scrolling in Ubuntu
http://ubuntuforums.org/showthread.php?t=2097030
But hasn't given me the answer. Just plugged in a mouse in 12.04 and there's no horizontal scrolling with the mousewheel. Instead it has strange behaviour where text is changed if mousewheel is held down and moved over it. HP mouse, feature works on windows 7.