Render > Ambient Occlusion > Compute now first prompts you to orient all your surfaces to face the same way before you start the calculation. Users often forget this step, resulting in incorrect occlusion calculations and bad visual results. The interaction to set surface orientation is the same as with Surface Edit > Orientation > Set Surface Orientation.

After unifying the orientation of your surfaces, click the Compute Occlusion button to proceed with the occlusion calculations. The model is flat-shaded with a plain white shader to better show the ambient occlusion.

Clicking Show AO Only toggles between the ambient occlusion mode (white) and the surface orientation mode (blue/yellow), where blue indicates surfaces facing toward the camera, and yellow indicates surfaces facing away from the camera.

In occlusion mode, surfaces for which no ambient occlusion was calculated appear red. Transparent surfaces display with their transparent shader (no ambient occlusion). The new Pick Uncalculated button lets you select all the objects in the scene for which occlusion was not calculated.