This video shows you how to get the apple that is hidden in Angry Birds Rio Airfield Chase Level 10 (9-10). The luggage towards the right of the level is labeled (under the tail of the wooden plane). Use the Yellow Bird to hit the gasoline can just to the right of the luggage and you will get your apple.
Note! With the v1.3.1 update Rovio flip-flopped level 9-10 and 9-11, hence why the level number in the video below shows “9-11″. The walkthrough is correct though, but the apple is now found in level 10, not 11.