To play machinist, you must unlock the heavensward (expansion) areas through the main scenario quest line. Once you get to the city of ishgard you can then unlock the 3 new jobs that were released with HW.

Also, a job is a specialized version of a class. There is generally only 1 job per class with the exception of arcanist since you can use summner or scholar with that. You can unlock your job at level 30 by completing your class quests at your respective guild and leveling an assigned ult class to level 15. For Pugilist you will do this in the city of Ul'dah and I believe the class you need to level 15 is lancer.

Classes and their jobs share level and allow you to unlock unique job abilities that are essential for end game content.

Here is a link to a website that explains it in more detail:

http://ffxiv.consolegameswiki.com/wiki/Jobs

Hope this helps!