Simply create the instance and show it, specifying a title, a message, and a stock icon, like so:

Glib::RefPtr<Hildon::Notification> notification = 
  Hildon::Notification::create("Something Happened", 
    "A thing has just happened.", 
    Gtk::Stock::OPEN);

std::auto_ptr<Glib::Error> ex;
notification->show(ex);
if(ex.get())
  std::cerr << "Notification::show() failed: " << ex->what() << std::endl;