Arceus isn't in the game at all, only a statue of him.
Unfortunately, even when Arceus was revealed before the production of Pokemon Mystery Dungeon: Explorers of the Sky, it is not a Pokemon which can be met or obtained in the game. There are no optional missions to encounter it or special events which make it available. There is only a vague reference to Arceus in the game, where Dialga refers to a Pokemon on "a higher tier" than itself.
Source and Source 2