you need to put the first ball in the right botton, then a ladder will appear, you go up, and you'll find another ball, you push it and then you put it in any of the bottons.
After doing that you should go back to the place were there's no floor, you can see something and you use the grapple, then the ball falls down.
if you want a guide you should do next:
in the first screen, were appears "PRESS START BUTTON" don't do that, in a moment, a demo will be played, put attention because in that demo, is the solution