Sounds like a nice project. Can you post the electrical circuit? or URL? With some explanation how the circuit works. The Arduino needs a 5 V pulse (opto coupled !) and must immediately start opening the shutter for some predefined time. Ideally the lightning must be in front of the camera.
There are several camera control sketches on the playground that can be a starting point for the code. See - http://www.arduino.cc/playground/Main/InterfacingWithHardware
What kind of camera do you intend to use?