Instructions
Obtain the Enigma Stone by using Action Replay, downloading it from a Wi-Fi Event or attending a Nintendo Event at a location, such as Toys"R"Us. As of this publication, Nintendo is not offering the Enigma Stone to "Pokemon SoulSilver" players, so Action Replay is the only way you can obtain it.
Teach Fly to any Bird-type Pokemon, such as Pidgeot or Fearow. Fly to Pewter City, Kanto. Go to the Pokemon Mart and pick up the Enigma Stone from the green Deliveryman. The Enigma Stone will go to the Key Items pocket of your Bag.
Head to the Pewter Museum, which is in the northwestern part of Pewter City. Enter the building and speak to Steven Stone of Hoenn inside. After inspecting it, Steven will suggest that you allow an expert to examine the Enigma Stone.