Hiya,
Sorry to hear you were having such trouble with it.
The first time I went and tried to register it said the same thing to me, that someone else had used that email address. Well I thought that was nonsense as how could someone have used that?? but I went ahead and used another of my addresses and got in fine.....
But recently I figured it out!! If you are already registered with disney.com or one of their websites you don't need to register for VMK, just sign in using your email address and disney.com password and you will be away

The person who has already used your email address - is you!
HTH,
Gaspodé