To access the control panel for a site, go to http://www.domain.com:2082, where "domain.com" is the master domain on the account. The control panel can be accessed via SSL at https://www.domain.com:2083, where "domain.com" is the master domain on the account. When accessing the control panel via SSL, the browser may prompt to accept a certificate tied to the server name. This is normal, and can be accepted without danger.
If port 2082 is not open, http://cpanel.domain.com can be used instead.