Hello MikeD,
If you wish to switch your service from Secondary to Primary, you'll need to access the Primary NS link within the DNS Settings tab:
Click Edit on the following page. Then check the box for "Delete Primary NS Settings" and you'll notice another helpful option appear called "Import Zone". Doing that will import your DNS records into our interface and Zoneedit will become your Primary DNS host.
To add a 3rd (or 4th level) domain name for DNS services with Zoneedit, you are required to send us a request. We will manually add the service to your account for you. In this case, you do not need to add your domain since it already exists in the system.
If you have any further questions or concerns, let me know.