Yes, I had the same problem. I bet your apple ID was NOT an email address? And that the email address associated with that apple ID is the email address you are trying to use to sign up for find my iphone? This has to be the case, as it was exactly what happened to me. You need to use a totally different email address, one that is not in use with another apple ID. You can login to at apple and manage your current ID and change that email address to something else and THEN use the email to sign up with find my iphone. The way this is working is really stupid, apple should know what this would be an issue.