In order to use a domain name with Tabulas, you much purchase a domain name from a registrar. GoDaddy.com sells them for $10/year.
Once you register a domain name, point the DNS A record for your domain to 67.15.181.21.
(Todo: Screenshots in GoDaddy)
After you've done this, email roy@tabulas.com with your new domain name and your Tabulas username.