//////////////////////////////////////////////////////////
//Code By cgiles
//Description : how to add mouse wheel interactivy
//////////////////////////////////////////////////////////
// default size of my ellipse
int tailleEllipse=100;
void setup(){
  size(640,480);
  frameRate(30);
  // this will add a new event listener to your sketch
  // for listen how the mouse scroll is used
   addMouseWheelListener(new java.awt.event.MouseWheelListener() { 
    public void mouseWheelMoved(java.awt.event.MouseWheelEvent evt) { 
      mouseWheel(evt.getWheelRotation());
  }}); 
}
void draw(){
  background(255);
ellipse(width/2,height/2,tailleEllipse,tailleEllipse);
}
// this is the event listener of mouse wheel
void mouseWheel(int delta) {
  // add the value of wheel's scolling to my ellipse's size
  // for a better effiency i multiply the scrolling value by 5
  tailleEllipse+=delta*5;
}

This is the way for listen the mouse wheel and interact from it in processing, enjoy

how to listen the mouse wheel in processing

Processing