Hello, try to do this way:
1. Mark all items you want to render in AO
2. Go under Chanelbox/Layer Editor and click on "Render Layer"
3. Under Render Layer you click the right Icon "Take the objects and assign to a new layer"
4. You have a new "layer 2" now, right mouse click on it, last option "attributes"
5. Here you can choose under presets the AO
6. Choosing this lets your objects get black because you assign automatically a surface shader to it
7. Staying on this render layer you can do this render and it should result in a AO render

Try also under youtube to search for AO renderings