Go back to Kamadan. Just east of First Spear Devad there will be a small dock just over a bridge and down some stairs. There are two people with quests there, one for Tyria, and one for Cantha.
Once you've done the canthan one, you should end up on a pier in Cantha, just talk to the guy with the "!" over his head, and head for the nearest portal and you'll be in Cantha.
