The Secret Key opens a secret room in Eterna City where you can change Rotom's form. This is the only way you can change Rotom's form, and the Secret Key is available only for a limited time.

The Secret Key is available via Nintendo Wi-Fi Connection April 20th through May 12th, and at participating Toys R Us locations in the U.S., Canada, and Puerto Rico April 19th through April 26th.

Source: http://www.pokemonplatinum.com/#/pokemon/rotom/