Você precisa instalar o xfce4-notifyd
:
Libnotify is an implementation of the Desktop Notifications Specification which provides support for GTK+ and Qt applications and is desktop independent: it is already used by many open source apps like Evolution and Pidgin. Libnotify can be installed with the libnotify package.
In order to use libnotify, you have to install a notification server.
Verifique o link
Comando de configuração: xfce4-notifyd-config