How do you get Arceus in Pokemon Platinum without hacking?

just look through all the pokemarts and Pokemon centers until you find a dude in a green suit. talk to him and he'll give you this flute. go to the spear pillar and select the flute and select "use". then it'll say it made a noise, then there'll be a staircase and go up the stairs to find arceus! it'll be about level 100, or 80. stock up with tough Pokemon before you go, and save before you go up the stairs! good luck!