It's a different NPC from the one in the ACN guild that unlocks SMN. If you have progressed far enough in the main story quest to have completed Sylph Management, you should have been sent to see her in Gridania at level 30 (guildmaster would have had two level 30 quests for you back to back: Sinking Doesmaga and then Austerities of Flame). Check your journal to see if maybe you logged Austerities of Flame without realizing it. If so, you need to go talk to the NPC in Apkallu Falls. Then your job quests will be with Y'mhitra for a while.

Here are some links to some info on it...first is an overall page for the job, second is the level 30 unlock quest--note the requirements section:
http://ffxiv.gamerescape.com/wiki/Summoner
http://ffxiv.gamerescape.com/wiki/Austerities_of_Flame