I hear this is how to activate it, although I have not been able to do it yet.
Anyways the way to activate it is you hold a grenade for 1-2 seconds then press Z wait until you die then release Z but continue to hold down + again wait until you die this should activate the code.... it took me a couple of times to get it right but after a couple of goes you should get it. Any more help needed contact me again.