If you get a notice that your email address is already in use when creating an account, that means that you created an account with us at some point. This usually indicates that you registered one of our notation programs, such as Finale or NotePad, in an old account under this address. For some people, this can happen if you create an account on one of our product pages, like http://www.finalemusic.com/, or http://www.garritan.com/ then try to do the same at another, like http://www.smartmusic.com/. All MakeMusic sites use the same customer database, so when you sign up for one, you sign up for all.
The best thing to do in either situation is to ask for the password. All login pages include a link that allows you to have your password sent to you. This way you can get access to your account and update it as needed. If you no longer have access to the email, simply contact Customer Success to have your account information updated.