Well, I guess I was the one that provided that "good-for-nothing" response;) Maybe it's time to fix this.
The idea behind that is really simple. Detect button click by connecting signal handler to one of the button's signals and pop-up the menu from there. The function you'll need is gtk_menu_popup()
and here is that tricky part: the GtkMenuPositionFunc parameter. If you want to position your menu just below the button, you'll need to calculate the proper position for menu.
Currently I lack access to machine with GTK+ dev. environment, so I cannot offer any more specific help. Try playing with thi stuff a bit and see what you can come up with.