Refer here: QUESTION
Those are the exact step I did in my install - a raring x64 server install + all needed xorg packages to run a GUI + MATE 1.6.
Basic lightdm + unity-greeter is flawless, as well as the other lightdm greeter available under ubuntu, and I also use Gala window manager from the eOs ppa, but pantheon-greeter fails to start from lightdm.
I am able to run it from a terminal window within a session, just never from lightdm - it always shows the message "unable to start job".