Applescript and Arduino
The USB cable used to connect your Mac to the Arduino acts as a serial port, and can be used for communication.  (For example, see Make an Arduino talk to your Mac).

Here is a thread that appears relevant: AppleScript serial communication with arduino

Or, you could go see this blog entry: Arduino-serial: C code to talk to Arduino.  You could compile the application [downloading xcode, first, if needs be] and then use it to communicate from applescript by using the "do shell script" command.