Ok, for people getting the stupid IP message, just keep refreshing the page and hitting the Get Key button. Eventually it will go through. I used a Mozilla Private window just in case (Chrome Incognito would work too) but I think it's just an error message for the site being overloaded again.
Edit: You still have to make a new account if you got one the other day, or else it will give you the same code as last time. New codes CAN be redeemed to the same MH account though--I just got Iron Man, which is exactly one of the ones I've been hoping to get.