Alright, I will give you a start and some hints...

this is a sketch of a moving rectangle.
As you can see i add the 1 ( the speed)  to the position as long as it is 100px away from the right side. if it is, i set the speed to zero. Now you have to add some mouseAction...
look at this reference Page:
http://processing.org/reference/mouseButton.html
think about how you can make it turn back again and how you would stop it if its on the left side of the sketch.
If you need further help, feel free to ask, but give it a try, its worth it.




float x = 100;
float speed = 1;

void setup(){
 size( 400, 400 );
 smooth();
 background(255);
 rectMode(CENTER);
}  


void draw(){
 background(255);
 rect(x,height/2,50,50);

 x=x+speed;

 if(x==width-100 ){
   speed=0;
 }
}