1) When you order a reseller account, you get the ability to use a custom name server. A custom name server is specific to your domain name, but uses the same DNS IP's as our DNS.

2) When a reseller signed up we set cpanel/whm up to use that name server so that on the customers account, and their customers account we are anonymous. The reseller name servers are given to you in your "Welcome/Setup Email"

3) Now, you need to go to YOUR domain registrar, login into the domain manager and you need to create those nameserver, they are also sometimes called hostnames. That will allow you to set up the IPs for ther nameservers/hostnames. Until you do that the nameserver will not ping.

4) Once you have created those nameserver/hostnames and assigned the IP's to them you can then change your domain over to use the nameserver/hostnames.
If you are not familiar with how to set this up in domain manager, you need to call them and ask them the following:
" I have a domain registered with you, and my website provider has set up custom nameservers/hostnames using that domains. I have the IPs I need to assign those nameservers to (ns1.yourdomain.com goes to xxx.xxx.xxx.xxx, and ns2.yourdomain.com goes to xxx.xxx.xxx.xxx) How in the domain manager do I do this??"
And let them tell you exactly what you need to do. If they tell you, you just need to change the domains over to use those nameservers they are WRONG. You first have to create the hostnames or nameservers and assign IP's to them, before you can actually change the domain name over to use them.