Since the writing of this article, the structure of the JSON message signatures on MyEtherWallet changed slightly. Here's the updated signature data structure that should work using the current version of that website:

{
  "address": "0x6954d0255cfed71d55ad8b371b4170b8bf7709dc",
  "msg": "I am Midnight426 on Steem and Twitter, I am MidnightLightning on GitHub, and I'm the owner of the Ethereum address 0x6954d0255cfeD71d55Ad8b371b4170B8BF7709dC, which signed this message.",
  "sig": "0xf515a622eff1f20e980216bc67306368c68685d84c15e6af6e8876c4de4a787d193c63cc786198ec1fec5932e54377764f3241fff03ecb805acef67d5844894e1b",
  "version": "3",
  "signer": "web3"
}