As someone who just unlocked Monk I can explain it:

Once you have the requirements met for the job you want (30 of main 15 of sub) you have to complete your MAIN class quest at 30. After you complete it you will see a guild receptionist that has a quest for you. Complete it, and you get the stone for the job. For a complete list of sub classes needed...here ya go.

Bard = 30 Archer / 15 Pugilist
Summoner = 30 Arcanist / 15 Thaumaturge
Scholar = 30 Arcanist / 15 Conjurer
Dragoon = 30 Lancer / 15 Marauder
Black Mage = 30 Thaumaturge / 15 Archer
White Mage = 30 Conjurer / 15 Arcanist
Monk = 30 Pugilist / 15 Lancer
Warrior = 30 Marauder / 15 Gladiator
Paladin = 30 Gladiator / 15 Conjurer

Or at least I think this is right.