you need to step on the pads in the correct order, starting from the pad with a pink cristal, to play a given melody. The image in the back should help to guess it. if you step on one of the cristals (the blue and green) on the side you'll hear the melody you need to "play".
is it a bit clearer this way? I'm going to write these instructions in the game page for others to see. Thanks for your comment!