The best way to get into the secret rooms in the Pokemon mansion is to find a cheat code for the Pokemon game and install it before playing. You should then be able to easily get into the secrets rooms on floors 2 and 3.
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The secret key is in the Pokemon mansion (Cinnabar Island) in the basement.
You simply can't go there.
if you got it from the Pokemon mansion it opens the cinabar Pokemon gym
The Secret Key that you get in the Pokemon Mansion is used to enter the Cinnabar Gym.
Find the secret key in the Pokemon Mansion,( Next to the gym)