IDArnhem: This code runs in Processing from your laptop and communicates with the Arduino using the serial protocol.

arduino_buzz.pde

import processing.serial.*;

int DATA_RATE = 9600; // must be the same as the datarate that the Arduino is using
Serial serial;

void setup() 
{
  size(200, 200);

  // the ard variable must be = to the location of your Arduino
  // say: /dev/tty.usbmodem1442
  // or something like that.
  String ard = Serial.list()[0]; // try to autodetect Arduino, if it fails, just hardcode the port
  serial = new Serial(this, ard, DATA_RATE);
}

void sendBuzzCommand() {
  println("Telling the buzzer to buzzzzz");
  // send the command 'B' to the Arduino
  serial.write('B');  
  // when we send a capital 'B' through serial from our computer, it will go 
  // to the Arduino which is programmed to react to that letter by buzzing the buzzer.
}

void draw() {
  background(255, 200, 200); // I like salmon-pink
  text("Press 'b' to buzz", 8, 18);
}

void keyPressed() {
  switch(key) {
    case 'B':
    case 'b':
      sendBuzzCommand();
      break;
  }
}