if your talking about the playstation 3 version then you cant unlock him unless you buy him for playstation store but yoda is only available for the XBOX 360. same thing for Darth vador you cant...
Yoda is available on Xbox360 and Darthvader is available on PS3
I do believe that you can now download yoda on the ps3, but he will come at a cost
Not sure if you can do the same on the xbox