While I am a Dragoon now, I wasn't always and I had a hard time finding a definitive answer about how to unlock the Job Class. At the time the threads I looked at were unsure about which secondary class to level to get the Dragoon class. So for all those who couldn't find the answer, can't find the answer currently, and to all those fellow Dragoons in the future, here is the answer you've been looking for!

1) Start the game as a Lancer lol
2) Level up Marauder to 15 (You don't have to complete the lvl 15 guild quest)
3) Level up your Lancer to 30
4) Complete your level 30 Lancer guild quest to unlock the Dragoon quest
5) Complete the Dragoon quest

As I mentioned before I am a Dragoon now and here is a picture to prove it. You're welcome. Nya! =^_^=