How do you get in the door at the hidden lake on clubpenguin?

First you have to be a member.

Then you have to play black puffle puffle rescue.

After you get the first puffle, wait for a big shadow.

Follow it, and when you are following it you have to stop at bubbles and be over them when they pop. If you don't then you will disappear and have to start over.

Then get the key and finish!