Please use http://cp.yourdomain.com(replae yourdomain.com with your actuall domain) and the username / password as given to you in your welcome email. If your domain has not propogated yet, you can use the alternative control panel address that is mentioned in your welcome message.
If you have not received the welcome email it could be because your account is currently going through verification. You can open a support ticket with us to verify your account status.