You can not encrypt a message using bitcoin signed message. You need PGP key. So the game is only for PGP key. You must add current date in the secret message you have for me and no one else should be able to read it except me [that's why it is encrypted]. I will post the decrypted message however If you feel uncomfortable then add it in your encrypted message, I will not post it to honour your request.
Please check your public key, probably missing a or some characters. In the link that you provided, I was able to use people's public key to encrypt message, but I was unable to use yours. Or try and encrypt a message with it to know if the public key is valid or not.