Instructions
Obtain an Arceus from a Nintendo event or the Hall of Origin. You also can get an Arceus by trading with a player who has participated in an Arceus event. The only purpose of Arceus in "Pokemon Soul Silver" is to give players access to the Sinjoh Ruins.
Trade the Arceus from your fourth-generation Pokemon game to "Soul Silver." You won't have to do this if you got a Pokemon player to trade Arceus directly to "Soul Silver." If you used Pal Park to trade, take the Arceus out of your PC and into your party.
Make the Arceus your partner Pokemon by putting it in the first slot of your party. The Arceus should now follow you wherever you go. The lead archaeologist will come across the Arceus, and he will ask you to visit the Sinjoh Ruins to uncover a secret.
Wait for the screen to turn black. Afterwards, you will find that you and the Arceus are at the Sinjoh Ruins. Head outside the ruins and you will see a cabin. Go inside and deposit all your Pokemon except for the Arceus into the PC.
Talk to Cynthia and she will escort you out of the cabin and back to the Sinjoh Ruins. She will show you the Mystri Stage. Choose whether you want Dialga, Palkia or Giratina. The Arceus will proceed to lay an egg of the desired legendary.