Depending on if you use the MenuFactory or not, there are two ways to proceed. With the MenuFactory, use something like the following:
menu_path = gtk_menu_factory_find (factory, "<MyApp>/Help"); gtk_menu_item_right_justify(menu_path->widget);
If you do not use the MenuFactory, you should simply use:
gtk_menu_item_right_justify(my_menu_item);