In Heartgold and Soulsilver, you don't need it. Put Rotom in the first slot and enter Silph Company in Saffron City.
Rotom will allow the elevator to work, and you will notice 5 items in the room:
Lawn Mower
Refrigerator
Fan
I think washing Machine
and
I think microwave oven
go to the item that you want Rotom to turn into.
and don't forget to read those notes in the room.
You can have all 5 forms of Rotom
6, if you count the original form.
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The card key in Pokemon Heartgold and Soulsilver is located in the Goldenrod Underground.
You can only get the secret key in Pokemon Platinum
The Secret Key that you get in the Pokemon Mansion is used to enter the Cinnabar Gym.
The secret key is in the Pokemon mansion (Cinnabar Island) in the basement.
You have to answer anyone you want. He doesnt give you anything.