You will need:
a Mac computer
AppleScript Proxy from Tinker It! get it here: http://tinker.it/now/2007/04/26/control-your-mac-from-arduino-the-easy-way/
Arduino. I used a Duemilanove
USB cable for Arduino
A photoresistor. Available from any electronics parts store. I got mine at RadioShack. Should cost about $3 for 5.
a 10k resistor. Color code: Brown Black Orange
LED if you don't have on on your Arduino. Though it's optional then, too.
Gather the above materials and install the AppleScript Proxy.