In order to obtain the Golden Egg #21, you need to get 3 stars on all Ham ‘Em High levels, this includes all 3 “Facebook Levels”. Once in the accordion level, expand it and hold it there until it breaks. No need to press any buttons actually.
Below you will find the video that shows both the location (or how to find it) and the starred golden egg level walkthrough itself.
Go Back To Our Angry Birds Golden Eggs Guide