After you rescue Wystan from certain doom, Momodi has a new task for you.
This quest is only available if you started the game in Ul'dah, you have completed the quest Way Down in the Hole, and have reached level 10.
Unlocking Other Combat Classes
Now that you are level 10, if you have completed your level 10 job quest for your current job (available in your job's guild), you can unlock other combat classes by talking to the Guild Receptionists in the various guilds in Ul'dah, Limsa Lominsa, and Gridania.
Starting the Quest
Talk to Momodi in the Quicksand in Ul'dah to start the quest.
Speak with Dadanen
Your next destination is in Western Thanalan. To get there, you can either go out of the gate that is north of the Thaumaturge's Guild in Ul'dah, or if you have attuned to all the aetherytes in Ul'dah, you can use one to go directly to Western Thanalan. Or you can talk to the Chocobokeep just outside of The Quicksand and pay a small amount of gil to ride a chocobo to your destination.
In Horizon, be sure to attune to the aetheryte. Then talk to Dadanen. There will be a cutscene.
Next Main Scenario Quest
After completing the quest, talk to Dadanen to accept the next quest: Supply and Demands.