External Trigger setup

Hello, we are trying to use external trigger to make the lightcrafter start to play images we download into it. We use a 3.3V power supply to connect the pin 2 and pin4(ground), so that if we want to trigger it, we apply 3.3v between pin 2 and 4.  In the software, we set the trigger type to be external trigger positive. But it does not work, would you please give any reason or suggestion why it does not work? many thanks!