You need to do the quest to be able to travel between continents. The quests needed to go to Cantha are :
* From Tyria you should acquire the quest : Mhenlo's Request from Firstwatch Sergio in Lion's Arch.
* From Elona you should acquire the quest : Plague in Cantha from Funwa Shento in Kamadan, Jewel of Istan
Please Note : Each of the two quests have requirements of their own in order to be available.
See
this page on the official wiki for more information about the quests.