I am level 15 now and I tried talking to Kreia but I didn't see an option for it. When do I get to pick my class?

Are you Light, Dark, or Neutral? Because if you're neutral Kreia won't give you the opportunity to gain a Prestige class. You have to be at least 75% Light or Dark in order to gain Prestige. It makes sense, really, because how can you become a Jedi Master if you're still playing with the dark side? Or how about a Sith Lord with a habit of showing mercy? Ultimately, you must choose!

 

If you are 75% Light or Dark, then I have no idea why you can't do it.

You also need Visas in your group, plus I have heard that you need to talk to Kreia outside the Ebon Hawk.

 

Hope you get your prestige class, but if not, it is no big deal...my first time beating the game I never got a prestige class.

You can get a prestige class inside the Hawk. And like Insinvin said, it doesn't matter whether you choose a prestige class...the game's easy w/out the prestige classes, and w/them you can sleep and beat the game (literally).

I was level 15, almost completely dark and got my prestige class from Kreia as soon as I finished on Nar Shadda and talked to her on the Ebon Hawke.