Configure a trackball under Linux without editing Xorg.conf
The new place to put tweaks is: /etc/X11/xorg.conf.d/10-evdev.conf which may be elsewhere according to distro, eg /usr/share/X11/xorg.conf.d/10-evdev.conf
For more info see the Arch Wiki for Logitech (there's a GNOME bug which is also mentioned in the Ubuntu writeup.)
The arch wiki also shows how to set it up per-user with ~/.Xmodmap or ~/.xinitrc
Use xinput -- start with list and list-prop options and work your way for available parameters -- like this for middle button emulation
$ xinput --set-prop "Logitech USB Trackball" "Evdev Middle Button Emulation" 1