This is pretty basic. In the subVI, Go to VI Properties and in Window Appearance, select customize and select Show Front Panel When Called and Close Afterwards if Originally Closed (or just click the Dialog radio button on the Appearance page). Then use an event structure to capture the Boolean value change event and put your subVI in that event. There is a shipping example called Dialog Box Demo that shows the old method of polling the Boolean and having the subVI in a case statement. some of the shipping examples for the event structure also demonstrate pop-up VIs. Look at one called New Event Handler.
