Add subclass:
You aren't eligible to add a subclass at this time.
To add a subclass, you must meet the following conditions:
First, you must obtain Mimir's Elixir. To find out more about this quest, speak with Magister Ladd at the Ivory Tower of Oren.
There is one important thing. You must also have completed the "Fate's Whisper" quest from Maestro Reorin, who you can find in a hut east of Oren Castle Town.
Secondly, your current occupation must be the second or third job and all of your sub classes must be at Level 75 or above. For example, if your current subclasses are Temple Knight and Silver Ranger, both Temple Knight and Silver Ranger must have reached 75 if you are to add a third subclass.
Thirdly, all characters are limited to three subclasses. That is, if any character has already three subclasses, he or she cannot add a new one.