Jincy: When you speak with the lady in Yayrie, it's the Southern Islet that actually opens up. It's midway between Islan and Hobo, on the bottom of the map.
However, of the Runes you have, which ones are illuminated (not black)? First, check to see if you can go to the Southern Islet. If not, let me know which Runes are still dark. If certain ones are still dark, you can't unlock the Southern Islet yet (but again, make sure it's not already unlocked first; that's where you should go next, if it is unlocked).
Briony:
Where the red arrow is, you have to walk across flashing stars over the void.
The other warps are just decoys.