The first step in rendering an Ambient Occlusion map for
your model is to change your renderer to Mental Ray.
You can change your renderer by clicking on the Rendering tab
at the top of the 3ds Max and going to Render Setup
or by hitting F10.
With the Render Setup window open, navigate to the Common tab, scroll down to Assign Renderer and click the button under Production.
With the choose renderer window open, select Mental Ray Renderer and click ok.
Now import the model that you want to bake an Ambient Occlusion Map to.
Assign a default material to it from the Material Editor
Now change the diffuse color to white.
With your model selected click on the rendering tab at the top of 3ds Max and select Render to Texture or by hitting the 0 key.
1. Change your Mapping Coordinates to Use existing Channel
2. Under Output click Add and select Ambient Occlusion (MR) and click on Add Elements
Under Selected Element Common Settings
4. Click on the button to the right of File Name and Type and to anywhere you'd like.
Next you can change you Map size.
5. Under Selected Element Unique Settings
Change your sample count to as much as you'd like (more is better)
Now your ready to hit render.
Now a new window should come up that looks a little bit like this.
