De link e link Eu entendo que você pode estar faltando
gsettings set org.gnome.settings-daemon.plugins.mouse active false
ou (alternativamente)
Then, once you run gconf-editor, from the GUI, navigate to
apps > gnome_settings_daemon > plugins > mouse
and uncheck the box for "active".
porque isso é
... not the end of the story, however, as Natty still uses gnome-settings-daemon to control mouse and keyboard settings. Usually, this is smart enough to get out of the way, but when it comes to mice, (specifically buttons) it will attempt to automatically ensure that your primary and secondary buttons are mapped to the system-wide left or right handed layout. In this particular case, we don't want that, so we need to turn this bit of functionality off.
Qualquer um dos itens acima deve funcionar. Deixe-me saber se isso não acontecer. Boa sorte!