Well, this is almost solved. I have an external monitor attached to my MacBook Pro. When I run my example it runs on the laptop monitor (which is on my right). And the dialog shows up on the laptop monitor on the top left of the screen. However, if I first move the main application to the monitor, then when I press the button, the Dialog is nowhere to be found! I can't find it on either monitor.
Does anybody know where the documentation for Tk::Popup is at? Or know what I can do to get the dialog displayed onscreen in this case?