Here is how to enter full servers on CP.
1. Log into Club Penguin as normal.
2. Click on a full server.
You should then see this message:
3. On your keyboard, press the button TAB until a yellow box appears over the word Okay. Then press the button ENTER.
4. Click Okay on the next message.
Then you should be in the Full server!
Go try it in hard to get to places like Mammouth
Credited to Tooly228