Sample code to use mouse wheel events in Processing.

// mouseWheel.pde
// code to use mouse wheel in Processing

// must import java.awt.event.*
import java.awt.event.*;

float y;

void setup() {
  size(400,400);

  // add mouse wheel listener
  frame.addMouseWheelListener(new MouseWheelInput());

  y=height/2;
}

void draw() {
  background(100);
  ellipse(width/2, y, 50,50);
}

// convenience class to listen for MouseWheelEvent
// modify mouseWheelMoved() for your own purposes

class MouseWheelInput implements MouseWheelListener{

  void mouseWheelMoved(MouseWheelEvent e) {
    int step=e.getWheelRotation();
    y=y+step*5;
  }

}