There is no option in Mouse & Touchpad Settings to enable horizontal scrolling in Ubuntu 13.04+ (Raring/Saucy/Trusty) (GNOME 3.8+):
No horizontal scrolling even by enabling two finger scroll or content sticks to fingers.
How is it possible?
There’s a way through dconf Editor; so:
Edit: Dconf Editor was pre-installed on 13.04, where I first faced the problem; but on 13.10 and 14.04 it’s easier to use gsettings, see Samuel Cavazos’s answer.
Or just run
in a terminal