You need to do the event (which is over now) but if you did the event, you go to veilstone city team galactic HQ and go to the wall to the left when you walk in. There should be a door. You go in (with rotom in your party) and walk up to a machine and press A while you're facing it
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Rotom can change forms in platinum
give him a secret key the different secret key the different rotom
its impossible unless you trade it
you don't need either but its easier to have rotom so you can get the rotom forms faster.
You dont you find him in different forms tell your freinds im only 8 and i now everything about pokemon my name is wyatt