In Platinum:
> Starting in Platinum, Rotom also has five extra forms (each based on a household appliance), which can be accessed by possessing specific appliances.
In HGSS
Rotom's forms were introduced in Pokémon Platinum by using the WiFi
Event item; Secret Key. However, in Heart Gold & Soul Silver, you do
not need this item to access Rotom's forms
If you take Rotom to Silph Co. and have it at the front of you team,
go to the Elevator and you'll be able to go up to it. Once there,
Rotom will power up the elevator and take you to the room. In the
room, select the respective item for Rotom's forms. You can change
forms at will. To change Rotom out of that form and back to its base
form, just select the tile where Rotom used to be
In Black Version 2 and White Version 2
Rotom's five alternate forms make a return within this game. When you
have sent Rotom over, you will be able to take Rotom with you to the
Department Store in Route 9. In this store, there is a room with
multiple storage boxes. Interact with this box and you'll be able to
change your Rotom's form. These Rotom all have special attacks that
can only be used while in that form and they change type.
In Omega Ruby and Alpha Sapphire:
#479 Rotom Location: Littleroot Town
In Professor Birch's laboratory in Littleroot Town. In the south-east
of the laboratory you will find some cardboard boxes. Interact with
these boxes and Rotom will change its form and learn a new move with
that goes with its form.
Source: Serebii
