That depends on what you want to do.
I asume you want to interface something to Flash via the Arduino board. Whatever that something is, determines what you need.
I have not used flash myself, but a couple of things you could do is to make Flash react to pusbuttons or switches, or potentiometers.
If you have not already read this, you should:
http://www.arduino.cc/playground/Interfacing/Flash
Good luck