[edited from follow-up]
Thanks for that reply. One more question - I did what you instructed to get MediaLab to call up Arena.exe and then the .def file for our virtual maze, but what seems to happen is that Arena opens to the point of having the file name selected in its start dialog box, but we still have to manually click Arena's "Start" button. Simultaneously MediaLab is displaying the message, "Click the Continue button to proceed." Is there any way I can change the MediaLab message at that point to tell the participant to click the Start button in the dialog box?