Arceus is Pokémon #493, the last Pokemon on the Heart Gold and Soul Silver National Dex. The only way to obtain Arceus is from the twelfth movie's event. The only other Arceus can be found in the Hall of Origin in Diamond, Pearl, and Platinum. However, there is no way to obtain this Arceus legitimately.
Arceus has a special event in Heart Gold and Soul Silver. When placed in the front slot of your party and brought to the Ruins of Alph, you are taken to the Shinjo Ruins. There, you meet Cynthia who brings you to a special room. Arceus has the power to create a level 1 Dialga, Palkia, or Giratina. Its your choice as to which one you want. The Unown will swarm, and the Legendary will be made. The Dialga will know Metal Burst, the Palkia will know Hydro Pump, and Giratina will know Shadow Sneak.