It is not intended that you write callbacks for this dialog. The Process Dialog already has callbacks written for it in the Programmer's toolbox code, you are only supposed to interact with it with the three functions:
CreateProcessDialog (load and display the dialog)
UpdateProcessDialog (update the percent complete and check for cancel request)
DiscardProcessDialog (Remove the Process Dialog)
If you want to check if the user has pressed cancel, you can do it with UpdateProcessDialog which returns a 1 if the user has pressed cancel and 0 otherwise.
Best Regards,
Chris Matthews
National Instruments