Instructions
Progress to the fifth world (Ham 'Em High) in "Angry Birds" and make it to level 13-12.
Send the first yellow bird anywhere on the map. Where you shoot this bird isn't important; it's the following bird that unlocks the secret content.
Launch the second bird, a white bird that drops eggs, in the direction opposite the targets. When the bird is directly over the backside of the hill on the far left side of the screen, tap the screen so the bird drops his egg. A large, golden egg appears.
Finish the level as per normal, then scroll all the way to the right on the main level screen until you reach the Golden Eggs screen. Tap "Golden Eggs" and then click the golden egg shaped like a football to uncover a hidden level that you can play as normal. Upon beating the level, a video trailer for "Angry Birds: Rio" plays.