I want to catch Arceus on Platinum. Is he an event Pokemon?
Well you would need cheets (action replay) because that event passed long time ago
Once cheeted walk into pokemart and talk to the guy in the green suit he gives you an item called the azure flute
Go to Speer pillar there when you step on the stone with a flute on the azure flute activates a massive pair of stairs appear and at the top is a lv80 arceus
You can't get up there again