When your reseller account is created, your nameservers are automatically created for you in WHM as ns1.yourdomain.com and ns2.yourdomain.com.
In order to properly use the nameservers you will need to register them with your domain registrar. Most domain registrars have an option for this as "Nameserver Registration" or "Register Nameserver". For additional help see the following knowledgebase article: https://my.hostmantis.com/knowledgebase.php?action=displayarticle&id=83
In WHM, you can see the IP addresses for the nameservers in WHM under "Basic cPanel & WHM Setup". Your nameserver IP's are also stated in your New Account Information email we sent you upon signup.
If you registered your domain name with HostMantis, you can register nameservers yourself in the client area:
Domains --> My Domains
Click "Manage Domain"
Click the "Management Tools" tab
Click "Register Nameservers"
From there you will be able to register your nameservers i.e. NS1 & NS2