Adding AO render pass in After Effects
Let's try to make it work. :D

1) Place 'Alpha' layer above 'Ambient Occulsion',

2) Change setting in 'TrkMat' of the layer 'Ambient Occulusion' to 'Luma Matte'

if Luma Matte is not working, try with 'Alpha Matte' (I'm not sure about your alpha layer :)),


3) Precompose 'Alpha' + 'Ambient Occulsion' layers ctrl+shift+c,

4) Set precomposed layer to 'Multiply' Mode.

Done!