How do you get to the place where Arceus is

He is found at Spear Pillar. Unfortunatly, he can only be released during events, which is when Nintendo allows peopleto be able to catch a certain form of/a legendary Pokemon for a short amount of time (All Rotom forms, Arceus, Darkrai, Giratina form, ect.) It should say on the official Diamond/Pearl/Platinum websites what and when their next events will be. You can get them over Wifi during events. It is also availage by hacking with Action Replay or some cheating device.
But, if there IS an Arceus event and you get the Wifi gift, you will get the Azure Flute. When you obtain it, goto Mt. Coronet (Where you caught Palkia/Dialga) And a staircase will come out. Before you fight ANYlegendary Pokemon, ALWAYS save your game if you intend to catch it. He will be at the top of the stairs. He is Level 80, so bring strong Pokemon