When you register a domain, the default nameservers are used. These nameservers are the default nameservers for the default server that a new hosting account is set up on.
This may or may not be the server that you wish to use the domain on. If an addon domain for an existing hosting account, use the same nameservers as the main account domain. A welcome email will be sent out for a new hosting account with the nameservers listed that need to be used.
If the domain is being hosted elsewhere, contact your hosting provider for the correct nameservers.