If you look at the things on the table around you that are not in direct sunlight or under a bright desk lamp, you'll notice that area closest to the objects are slight darker than further away. This is because the object prevents light from hitting the table from all directions. So, the light has been blocked. This blocking is also called occlusion. 

This effect is very dependent on scene scale. My coffee cup is blocking light and darkening the table top around it within about 2cm or 1/2 inch. So, if I was going to imitate that in Carrara, I'd need a "small" scale scene and an occlusion distance of  2 cm or .08 ft approx. 

Ambient occlusion in Carrara is done by telling the program to simply darken  any ambient light within x distance of the objects. If the number you choose doesn't reflect a realistic distance for the scale of your scene, it won't do anything for you.

Mark