Your main() function must do some extra initialisation before using the Hildon::Notification class. You must call Hildon::notify_init() and it is currently also necessary to initialise D-Bus, like

Hildon::notify_init("Notification example");

DBusConnection* conn = dbus_bus_get (DBUS_BUS_SESSION, NULL);
dbus_connection_setup_with_g_main (conn, NULL);