In Platinum:
First you must find the Secret Key via Wi-Fi Connected Mystery Gift. Once you have it and Rotom in your party, got to the Team Galactic Hideout in Eterna City (provided they're not there anymore) and go to a room in the Northwest on the first floor to use the Secret Key on a bookshelf. This bookshelf should disappear and behind it you will find a room. In that room, there should be five household items. The refrigerator is the one Rotom should possess and become Frost Rotom with.
In HeartGold/SoulSilver:
Put rotom as the first Pokemon in your party. Then go to Sliph Co. and use the elevator(Previously it would not let you in) Then you will see 5 appliances. The rest is the same as in Platinum.
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Rotom does not evolve at all ,but in Pokemon platinum (which will come out in spring 2009)it has 6 forms. Original Form, Frost Form, Spin Form, Wash Form, Cut Form and Heat Form. i also heard it evolves into somthing called rotomore i do not know if this is true that is just what i was told
Rotom is an Electric/Ghost-type Pokemon that is #479 in the National Pokedex. It has the ability to change in multiple forms - Heat Rotom, Wash Rotom, Frost Rotom, Fan Rotom, and Mow Rotom - that all have different secondary typing in addition to Electric.
Not everyone agrees on the best Rotom form, however, the most used form is Rotom-H. The second most used form is the Rotom-W.
i think u a secret key before you can change rotoms formes
In Pokemon Diamond and Pearl and Platinum which are the newest versions on the DS until the newer ones you catch Rotom after you get the national pokedex. Then you can transform him into different types.