The tooltips are not showed when they are associated to a Gtk_Menu_Item
contained in a Gtk_Menu_Bar. Unfortunately no example code can be provided
in C because I am developing in Gtkada (binding of Ada to GTK):

  Vbox1 : Gtk_Vbox;
  Menubar1 : Gtk_Menu_Bar;
  Menuitem1 : Gtk_Menu_Item;
  Label_Menuitem1 : Gtk_Label;
  Menuitem1_Menu : Gtk_Menu;
  New1 : Gtk_Image_Menu_Item;
  Tooltips : Gtk_Tooltips;

begin

[.....]

   Gtk_New (Menubar1);
   Pack_Start
     (Vbox1,
      Menubar1,
      Expand  => False,
      Fill    => False,
      Padding => 0);

   Gtk_New (Tooltips);

   -- In this Gtk_Menu_Item the tooltip is not showed
   Gtk_New (Menuitem1);
   Gtk_New (Label_Menuitem1, "File");
   Add (Menuitem1, Label_Menuitem1);
   Append (Menubar1, Menuitem1);
   Set_Tip (Tooltips, Menuitem1, "F1: This is the File menu");

   Gtk_New (Menuitem1_Menu);
   Set_Submenu (Menuitem1, Menuitem1_Menu);

   -- In this Gtk_Menu_Item the tooltip is showed
   Gtk_New_From_Stock (New1, "gtk-new");
   Image_Menu_Item_Callback.Connect
     (New1, "activate",
      Image_Menu_Item_Callback.To_Marshaller (On_New1_Activate'Access), False);
   Append (Menuitem1_Menu, New1);

   Set_Tip (Tooltips, New1, "This is the New menu item");

[.....]

end