Arceus can be found in Heart Gold or Soul Silver,One way would have been from an event in Diamond, Pearl or Platinum. As these events have already happened, this means that you’ll need to trade with someone who already has Arceus.
Another alternative is to use cheats, but this can sometimes be risky, so make sure you get yours from a reputable site, or someone you trust. Ones I recommend are Game Shark or Action replay. If you have any of those, find out the cheat codes for Arceus, and press L+R to generate the code. Then you go to the Violet City Pokemart and a guy will give you Arceus
Here’s a good site for you to have a look at: www.serebii.net/heartgoldsoulsilver/arceus.shtml Here, you can learn all about Arceus in Heart Gold and Soul Silver.
Heart Gold and Soul Silver are remakes of the 1999 video games Pokémon Gold and Silver, and are part of the Pokémon series of role-playing games. They were developed by Game Freak and published for the Nintendo DS, first in Japan in 2009, and then in North America, Australia and Europe in 2010.
The idea behind the games was to give a feeling of continuity for the people who had played Pokémon before, but to give those who were quite new to the Pokémon concept a different feel. The games were very well received, in fact both of them are among the highest-rated for DS games, and now hold the title of the highest-selling handheld games of all time, with 10 million units having been sold by the end of July 2010.
Heart Gold and Soul Silver are in the Johto region of the fictional universe, and of course, feature special creatures called Pokémon. The aim of the game is to become the best Pokémon trainer in the Johto and Kanto regions by defeating other trainers. The games are bundles with the peripheral, Pokéwalker, which connects to the Nintendo DS game card through infrared signals.