Instructions
Enter any area of the Chao Garden. You get to the Chao Garden by either selecting it from the "Stage Select" screen, or by collecting a key in any of the stages. Collecting the key takes you to the Garden when you finish that stage.
Press the "Y" button to change the commands at the top of the screen. Keep pressing "Y" until the "Whistle" command appears.
Press the "B" button to whistle. Any Chao in that area of the garden comes running to you. This is very useful if you are trying to find a certain Chao, since Chao tend to fall asleep under rocks and in plants, making them hard to see.