Thanks for the reply. I can't use the method from the tutorial because my scene includes tree (so I have to apply a special AO material to it with alpha channel to have the leaves appear correctly). I have been trying to do it some time ago, but after spending too much time on it I gave up and just recently went back to it.. you can see the original post here:

https://simplymaya.com/forum/showthread.php?t=36842

I am now able to create the materials (two of them: specific one for the leaves, and regular for everything else). I can see the final image just fine, but I don't want my scene lights effect the AO in any way, so I turn them all off (so that my work area is all black), this should not affect the render, but it does. The render is all black.

I can turn of casting shadows on all my lights, but wouldn't the light still affect the AO? I want the same amount of light everywhere in the scene..