There is no way to get Arceus in Platinum legitimately anymore. There was an event for Arceus (for DPPT), but it was never released. The code for the event is still in the game, but it can only be activated with cheat codes (which can only be used with a cheating device). There was an Arceus event (or multiple) for HGSS. You could trade that Arceus to Platinum, but that's not an option for you. You're going to have to find other ways to get Arceus... In newer Pokemon games. You can always use Giratina :P. Good luck! I hope this helped!
Edit: KFCutMan: As an alternate note, there were distributions for Arceus aside from the Azure Flute, but those have long ended by now. The only way to get an Arceus in Platinum now is to trade with someone who had the event, or using a cheating device for it.