You will need to complete a few quests for one of the factions until you are able to unlock level 5. Once you do, when you talk to the quest giver and look at their offers it will be the upper left option to buy an promotion. It will show unchained (unlocked) once you have earned enough reputation with the faction to unlock it.
Apparently (as mentioned in the comments below, thanks peeps) you can earn rank by earning money, selling items like explosive barrels. You can also earn experience by purchasing 5 voyages. If you wish to do so quickly you can purchase 3, propose them in a solo game and then cancel so you can buy 2 more. Personally...if you are going to play this game, this should just pop naturally you don't really have to "work" at it. That is if it doesn't go buggy on you.
Hopefully the bug gets fixed for those of you suffering w/ it.
Good luck out there!