How do you get Arceus on pokemonpearl

You will get a letter from Prof. Oak, telling you to go to route 224. I think you need the Walk-Through-Walls cheat to get there. When you get there, talk to Prof.Oak, and he should give you a Flute. It is Bluey-Purple. When you get this, go to Spear Pillar(where you catch Palkia), and go to the centre of the Pillar place, and blow the Flute. A hidden set of stairs should appear, go up them and you should find a Arceus!