Where's the Rotom Room in Platinum?

Question

I heard that you change Rotom's form in a room in the Eterna Galactic Building. Where is this room and where's the secret key that unlocks it?

Answer 1

The Secret Key (Japanese: ひみつのカギ Secret Key) is a Key Item. It is shaped like Rotom's head, with the outline of its eyes and mouth visible under the coat of Rotom-orange plastic. The key's teeth look like the prongs of an electrical plug.

Having the Secret Key and taking it to the Galactic Eterna HQ will open up "Rotom's Room", a special event area only triggerable by having the aforementioned item. Inside the room you will find appliances (Fan, Washing Machine, Mow, Fridge and Oven) that allows you to change Rotom to its respective formes.

The Secret Key item is (legally) obtainable via the Nintendo WFC or in select stores in certain countries during a time-limited distribution period, which has passed by now by the way, so there is currently no legal way to obtain the Secret Key item (without using the DNS Exploit, which can work on this)!

Hope this helps! You can also find further information regarding the event period in the source cited below.
Source

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Credit to shinkaigyo for the answer. Hope you're ok with this.

Answer 2

You must first obtain the Secret Key in order to get to the secret Rotom room and you have to go to Team Galactic's First Hideout in Eterna City and where the in the middle of the bookcase and wall is where is the Secret room is at use the key and it opens it up