void ShowDialogWindow(Type viewType, DialogInteraction dialogPayload);
This method creates a new dialog box, the contents of which is the text you supply in the viewType parameter. The UDI Designer creates a new instance of this type and wraps it in a dialog box that has OK and Cancel buttons.
You pass data to your control using the dialogPayload parameter. The SampleEditor solution in the SDK directory has an example of how to use this functionality.