question: applying AO render to texture maps

what is the proper way to apply an AO map for the render to texture roll out?... i've made sure my renderer was on mental ray, but there are a few MR settings and i dont know if i'm using the correct one, but the result i get every time is black poly clusters... I took the map into photoshop to see if i should just invert the map... so i could see the soft black shadows u generally would get from rendering out a shot of your model-- as if the shader were actually on it... havent been able to get it to work... and would you put this in one of the map slots? or would you use this map in terms of a shadow template for the diffuse map? what's the best way to use it?.. thanks