I had this feature in 11.10 after using a tweak by Ubuntu tweak. The new version for 12.04 seems not to have this tweak. How can I get this tweak?
You can achieve that with CompizConfig Settings Manager.
Install CompizConfig Settings Manager:
Hit Alt+Ctrl+T to open terminal and run following command:
Or search for "compizconfig-settings-manager" in Software Center and install it from there.
How to set it up?
So far, after having the same problem with 11.10 upgrade to 12.04, that this feature was unavailable.
What I tried was
in a terminal window. This added the scroll wheel to the viewport switcher.
Afterwards, this still did not work fully for me, but it's worth a shot.