How do you get the key to the door in the hidden lake on club penguin if you are not a member?

You can't. It's one of those things were you MUST be a member in order to do it. I've tried, sometimes it glitches or you can hack it. But I don't recommend trying because they have fixed most of the glitches and hacks such as the 1,000,000 coin hack from the DJ game. And I've gotten banned from certain servers from trying... '-_- but if you want to try, good luck.