Zac has the right idea.
Lightmass AO is mostly intended to be used for non-lighting effects such as placing moss or moisture in the corners of intersections of rocks. It is a method to get a more ‘custom’ painted feel for meshes that have been jammed together. It was not really intended to take that node and plug it into material AO, but there should also be nothing stopping you from trying it. But if you rebuilt the lighting, the lightmass AO was already used internally by the static lighting.