Assign that set to all geometry in scene. You can use multiple sets, to optimize calculation for different parts of your scene. I used only one set. Select all geometry. Then go to rendering menu set and go to lighting/shading / batch bake (Mental Ray) option box.

Skip objects in initialBakeSet means that objects that are not in any bake set you created will not be rendered. If you uncheck keep original shading network, surface shader will be assigned to objects. You don’t want that. So hit convert and close. This may take hours, so be patient.

AO map is rendered in renderData/MentalRay/Lightmap subfolder of your project folder.

Map shall look like this.

Then create file node and connect it to ambient color of materials.

If you render now, you will get something like this. AO texture is using wrong UV set. It’s using map1 (default uv set) instead of BakeSet. This is normal, since every texture uses this set unless you specify differently.

So select ground surface and go to window/relationship editors/UV linking/Texture-Centric

On the left side is your Material and on the right is surface. You can notice that file4 (AO texture) is linked to map1. Just click to BakeSet, and texture will use this set for that surface.

Repeat this step for Bridge.

In hypershade you can see new node called uvChooser1. Picture shows network for one concrete material for the bridge.

And second concrete.

If you render you can see that Ambient occlusion works.

I also multiplied AO texture with sky blue ramp to get tint.

So shading network for all materials share one single MultiplyDivide node

So image looks little better now.

* Mental Ray ambient occlusion is very slow even on simple scenes like this. You can use the power of your GPU to calculate AO.

There’s program called Faogen that uses GPU to render AO. To use it create copy of your scene, and copy uv’s from BakeSet to map1 set, and export scene to OBJ.