How to earth a MagSafe2 adaptor for 15" MacBook Pro with retina display?
The ground / earth is comprised of the outer-most two pins in the MagSafe connector (both original and the thinner + wider MagSafe 2) and requires that the ground be connected to the other end of your AC to DC adapter.



Rather than using a two prong plug, you could use a longer two prong cord and then ground the metal disc that serves to hold the "duck head" on the transformer/rectrifier/filter that makes up the bulk of the MagSafe adapter.