Either cheat for it or trade with a friend.
pokemon events, or trade the event pokemon from a diamond/pearl/platinum game.
IMPOSSIBLE but beat me and I'll give you my arceus 4296-8949-1934.
They give you secret items which you can only get through these events which get you secret Pokemon suchas darkrai jirachi shaymin and arceus
both through a event to to serebii.net for upcoming events and all things Pokemon and a little Mario
Arceus isn't available in the Pokemon versions of heartgold/soulsilver. It can be obtained from trade with another person (wirelessly). you can also get it from Nintendo events from older versions such as diamond/pearl/ platinum. good luck ;)
It's impossible to get without certain events/Hacks. Sadly, all of the events are over.
All Legendary Pokemon you can catch in Platinum: Giratina, Azelf, Mesprit, Uxie, Dialga, Palkia, Heatran, Regigigas, Cresselia, Darkrai, Articuno, Zapdos, Moltress, Regirock, Regice, Registeel, Shaymin, Arceus. (Darkrai, Shaymin, and Arceus can only be got by events).
You can only get Arceus through Nintendo events if you don't want to use cheating devices.
You can catch Arceus in Platinum with the Azure Flute which is obtainable through Official Nintendo Events but otherwise, you will not usually be able to get an Arceus from someone.
I think empoleon is the best when it comes to starters in platinum because he has a great defense and a really high special attack. Other then that, Giratina is the best of platinum, especially in the origin form, with the right moves he can destroy anything.
Only from events.
It wont appear because Action replay codes such as events don't work for that game. I have Pokemon platinum and only a few codes do work, you get the event item but they do not work.