Look at this:
(sorry for poor quality, I recorded it with half the size of desktop res, and youtube made it even worse)
Don't follow me with the curves option, I was trying to be quick with it, and its turned out very bad, but thats not the point here. To edit diffuse map you can use curves, exposure and\or shadows\highlights. This is vital to do it right. For example: on the video Ive posted, after curves edit, most of the texture was too black, so once pasted into AO and saved you will see that its covers too much, and is too solid. So, do it better than me

There is also other method. If I remember correctly, danlisa in his tutorial was suggesting to ctrl+click on blue channel, then copy and past to alpha, but that leaves out all those darker areas... but you can try this too, maybe you will get better results than me.
If you are using your own ao tex, it may be different in that it may be in alpha channel; in that case copy the content of alpha channel to rgb, and then proceed with above (diffuse map to alpha channel).