Processing is best used with Arduino for receiving serial data.  Here is a Processing example for a simple read over the serial port.

A simple way to test the serial with just your computer is to get 2 USB->Serial connections (RS232), a serial cable and connect them together (USB/Serial-->SerialCable-->Serial/USB) and then you can output and read serial information.  This may help in testing Processing code if an Arduino is not available (as is in my case; waiting for it to arrive, but have an assignment due before then!)  Here is a simple serial read on Processing.

Name	read()
Examples	// Example by Tom Igoe import processing.serial.*; Serial myPort; // The serial port void setup() { // List all the available serial ports println(Serial.list()); // Open the port you are using at the rate you want: myPort = new Serial(this, Serial.list()[0], 9600); } void draw() { while (myPort.available() > 0) { int inByte = myPort.read(); println(inByte); } }