It's on the extended dvd version, although I've heard that in some places (the UK?) that it had to be taken off. It's on the first disc. Click on "Select a Scene",
go down to the last one (I think it's chapter 30), then click again to get to
"New Scenes" and a Ring will appear. Click again and it's on.
(The Easter Egg does give you a greater appreciation of the stress Andy
Serkis must have been under in the filming).