The sensor goes to your mouse. Make sure the sensor doesn't touch the purple wire! Get to green to move on to the next level. I recommend you to attach a mouse. (If you have one attached already, then great) TIP: If your mouse is in the center of the sensors, be sure to have it ahead of then instead or the sensors will stop moving!
Thanks @Hemkernator for the settings button. All i did was change the color. NEW THINGS IN v1.1 - Added a settings button - Added some new levels and more levels - You can now change your background color - Added lives - Added a 1.5 second pause before you start the next level. - Little easier to get to the next level - New Thumbnail