If you get a 'User Does Not Exist' or 'Email is not a valid email address' error, the account has either not been signed up, activated, or an email change process has not completed.
You can sign up here; once done, check your inbox (and spam folder) for an activation email from noreply@mailer.selleramp.com. Click the link in the email to set credentials and log into SAS.
If your subscription has been activated and you're trying to change your email address, check out the steps here.