How do you get the Secret Key from Pokemon Mansion?

You can't get a "Secret Key" in Pokemon Mansion. There's no such item in the game.

Here are some things you might be thinking of:

* The Secret Key in Pokemon Mystery Dungeon: This is a different game and doesn't relate to Pokemon Mansion.

* The Golden Key in Pokemon Red/Blue/Yellow: You can get the Golden Key in these games after defeating the Elite Four and obtaining the National Pokedex. It's required to access the Kanto Safari Zone.

* Keys in Pokemon Mansion: Some Pokemon Mansion rooms require keys to unlock. However, these are usually specific keys you find within the Mansion.

If you're having trouble finding a specific key in Pokemon Mansion, please clarify which game and location you're referring to.