You will need some software on the Mac that can read the serial port and start the MP3 file playing.
Arduino nows nothing about Mac's / PC's / folders / files and all that.
It's easy to set up the photoresistor as a voltage divider and use Analog read on the arduino board to get a "light level" reading. Then send something over the seral line to the Mac, but theres no way the Arduino board can start playing a MP3 file on the Mac. Some program on the Mac must be listening on the serial port for what the Arduino board sends and take action.
thank you! I think I got it now.