El Captain - Do not see NAT 64 option?

It seems that we have to press Option Key while pressing Internet sharing when selecting the Sharing panel options on the left hand listing. 

On Windows keyboard, this seems to be Ctrl Key. With this, the NAT64 check box appear below the window. 

Btw, this option does appear only on El captains and not anything below this. 

references:

https://developer.apple.com/library/ios/documentation/NetworkingInternetWeb/Conceptual/NetworkingOverview/UnderstandingandPreparingfortheIPv6Transition/UnderstandingandPreparingfortheIPv6Transition.html