Every Isolta's user has their own username and password. Username is their email address. Because every user has to have a different username, you can only have one Isolta's account with one email.
You are getting this error message because you have already registered as an Isolta's user. That is, you can not register with the same email address again, but you can use your previously registered account.
This way you can log in to your account:
Go to www.isolta.com
Click “Log in”.
The system will ask you to enter your username and password. Username is your email address and the password is the password you set while you registered previously.
If you don't remember your password click "Forgot your password?"
Enter your email address and click "Change password".
Check your email. You should find a message from us regarding the password change. Follow the messages instructions.