You can prep the DNS zone from: https://support.hostrocket.com/clientarea.php > Domains > My Domains > Manage Domain section, you should see an option for DNS Management where you can create DNS records as necessary. If you do not see the DNS Management option and would like it enabled, please submit a ticket.
Once you have the DNS zone prepped, you would then set the nameservers for that domain to that of our registrar. You would do so from the client area > Domains > My Domains > Manage Nameservers next to the domain in question, then update the nameservers to the following:
Only then would the DNS zone entries that you created have any effect.