In order for your nameservers to work properly, they must be valid, registered nameservers. Therefore you must register them with your domain registrar. The following are some how-to's to registrar nameservers with a few of the most popular domain registrars:
The IP address for ns1.yourdomain.com will be the same IP address as your dedicated IP. For the IP address for ns2.yourdomain.com you can use the main server IP address. For any questions regarding namervers and/or IP addresses, just contact us!
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