The following illustration is in jpg format, for those using obsolete browsers.
Sit a comfortable distance from the screen and look--don't stare--at the dividing line between the two images. Let your eyes relax (as if gazing into infinity), until you start to see three images. Pick a convenient object within the scene as a reference and gradually bring the two parts of it together until the center virtual image pops into three dimensions. Moving your head nearer or further from the screen sometimes helps.