You have to get the Eon Ticket. You can obtain it via StreetPass on the BuzzNav. It will allow you to take the ferry to the island where you received Latias/Latios and capture the other one. If I am not mistaken, it will also be holding the Soul Dew.
EDIT: It turns out that the Soul Dew will actually be behind [email protected] after you catch it. But you get the gist (thanks KingArcanine).