If you have the job stone, it's in your armory chest (open your character equipment page, look to the bottom right under your rings).

Double check you've done all the steps otherwise:

Finished MSQ to the correct point,
Finished all CNJ quests,
Received the Job stone,
Equipped the Job stone
At the correct spot/with the correct NPC.

(Typically there will be info pointing to your next job/class quest on recommendations and/or below the next MSQ pointer on your HUD)