I presume you’re in Photoshop or PSP or GIMP at this point?
There’s a few ways to get the AO pass out of the alpha channels.
My preferred way is to render all the layers out as BMP straight from Carrara and then re-assemble them as I need them. All the alpha layers are produced as greyscale images which I can use as masks.
However, seeing as you already have an alpha layer;
Set up a new layer group with two raster layers in it,
fill one layer with black and the other with white.
Load selection from the alpha channel and select the AO layer.
Now delete/clear the newly selected region in the top layer of the group.
You can then consolidate the group to get a greyscale image, you may need to invert it if you have gotten it backwards.
You can also do this to a single black filled layer but remember that it will end up partially transparent.
You could load the AO layer as a mask in the group as well, which pretty much does the same thing.
Extra tip. I’ve mentioned this before but the AO is a pure geometry effect and you can produce it with no lights on in the scene at all.
Switch off all the lights including ambient, make sure the only layer selected is AO and render out as BMP. You can get a very fast render this way even with high quality settings. I leave you to experiment yourself on how to optimise a workflow using that fact.