If the domain name your using for your reseller account is with another provider, before you start adding clients accounts you need to create your host nameserver records. These are created by your domain name provider.
Your domain name provider may have a control panel where you can do this, if not, you will have to contact them to request they do it for you. Your account activation email contains the two unique IP addresses that you are to use for you host nameserver records.
Host nameserver records can take up to 72 hours to be created, once this time has past you need to point your reseller domain name to your new nameserver records.
For example point your-reseller-domain.com to:
As soon as they are complete you can start adding clients accounts to your reseller account. Remember that your clients need to point their domain name to your reseller nameservers in order for it to work, nameserver changes can 24/48 hours to fully propagate across the internet.