I believe i read somewhere you need to do a Shao Kahn mission or something,sorry i cant be more elaborate then that.

If you go to Kahn's encampment, with the white building with Asian-style roofs, talk to Kahn and he tells you to find Reptile. When you talk to Reptile I think he says he lost something Kahn wanted I don't remember exactly), so he doesn't return to Kahn. When you return to Kahn, he sends you on another mission, which is rather amusing...although you can't complete it on your first trip to Outworld.