How to get event Pokemon?

[Jaerri]

No, you don't need to have your 3DS with you (or the ID) just go there and ask for the Serial Code He then gives you a piece of paper where you can scratch the code free. 

"The Mythical Pokémon Darkrai will be available at participating GameStop stores starting May 1, 2016. Visit GameStop to pick up a code that will unlock Darkrai in your Pokémon Omega Ruby, Pokémon Alpha Sapphire, Pokémon X, or Pokémon Y game. Be sure to redeem your code in your game by August 31, 2016!"

There's even a link to a guide on the page! :D
http://www.pokemon20.com/en-us/mythical.html#may