How do you get the key to the door on club penguin in the mermaid cove?

To get the key to the door on Club Penguin in the Mermaid Cove, you have to be a member and play Puffle Resuce. Then, choose the black puffle. Then, you stay where you are. Youw ill see a huge shadow, you must follow that and collect the air bubbles. It will lead you to a green area, then follow it until you find stiars. Onc eyou are down them, you collect the old key.