All you have to do is get the Eon Ticket. After you have received the Eon Ticket through event or StreetPass, you can speak to Norman in the Petalburg City Gym. He will explain the ticket and take you to the ferry to go to Southern Island. When there, you will find Latias in Omega Ruby or Latios in Alpha Sapphire, opposite one to the one you were given earlier in the game. After capture, you will find the Soul Dew on the ground.