How to unlock a job at level 30

Short Answer

You need to have done MSQ level 20, called "Sylph Management" for the job advancement quests to appear.

So just continue doing your Main Story Quests!

You can easily see when it's available by looking at your Main Scenario tracker. The yellow ! bellow the tracker indicates your job quests.

Almost everything in this game is gated by the Main Scenario Quest.