Help me change my ac filter
 

There is a filter lying there on the bottom but I'm not sure if thats where it's supposed to be. I've divided the pic into quadrants for easier explanation.

Is there a hole under the filter where air can enter? (Look underneath the furnace).

if the return duct is attached to the bottom and not the sides that is where the filter goes.

That's just like mine. That filter cleans the air coming back thru the cold air ducts to the bottom of the furnace. My furnace sits on a manifold raised off the floor and the cold return air is ducted back to this manifold. Just pull the filter out and put a new one in. Mine has a steel wire bent to hold the filter down because the suction lifts it up otherwise.