The Slave/Backup DNS zone also known as Secondary is the read-only copy of the DNS records. This means that the DNS records cannot be added directly to the Slave/Backup DNS zone. The Slave/Backup DNS zone can receive the updated records only from the Master zone of the DNS server. The records in the Slave/Backup zone can be managed only at your master server. The Slave/Backup zones are available only for premium customers.
Login in your account, click on the DNS zones [Add new] link, click on the Slave/Backup zone box, enter the domain name (without www or http://), enter the Master Server IP and click on "Add slave".