Where can you catch Arceus?

Answer 1

first you need to get the azure Flute from a Pokemon event. then you go t the spear pillar and stand on the tile with a flute on it. play the flute and a staircase will appear in front of you. climb up it and he will be there!

Answer 2

you can catch arceus in diamond/pearl where you caught dialga or palkia using the flute you can get from an event or you can get the flute using action replay.

you can do the dreamworld event for arceus in black/white. (diamond/pearl/platinum event finished)

Hope this helped :)