This is how to get the Secret Key to unlock Rotom's many forms, step by step (information obtained from http://www.pokemonplatinum.com/#/pokemon/rotom/)
Talk to the Tv Producer on the 3rd floor of Jubilife Tv Station
Answer his questions with these words:
Opinion on Tv: Everyone Happy
Feedback on Tv; Wi-Fi Connection
Then save and switch off your DS
Switch your DS back on go to the Platinum start menu. I there's a Mystery Gift option, you've succeeded! Select the Mystery Gift option to get started.
Select 'Receive Gift', then there should be three options. Ignore the 1st option that says 'Get From a Friend'. You can only use this if u have a friend that has already obtained the Secret Key. Also ignore the 2nd option 'Get Via Wireless'. You can only use this if you are near a special place where they give out the Secret Key(which is very hard to find and I dont think they are availabe anymore so dont use this option). So the only other option is to use Wi-Fi, so sorry if u dont have it. After you've selected 'Get Via Wi-Fi' and received the Secret Key, go to any Poke-Mart and talk to the man in green in front of the counter. He will give u the Secret Key which u can view in the Key Items pocket.