When you register your domain name, you will be prompted to enter your domain name servers (or DNS Information). If you already have your domain name registered, you will need to login to your administration section of your registrar's website and change your dns/domain name servers there. To point your domain to your Charlottezweb account, you need to alter this per the signup email you get from us.
The formatting would be similar to the following:
ns3.charlottezweb.com
ns4.charlottezweb.com
Note: The actual number values vary, so consult your signup email or contact us for support if needed.