When you register to freehostia use this option ''Use my existing domain'' and enter your domain name, when you receive your Account info you should also receive the DNS, it should be dns1.freehostia.com & dns2.freehostia.com, now head back to the domain registrar and change the DNS server to dns1.freehostia.com & dns2.freehostia.com
I also had the same problem when I registered the free domain with co.cc, and so I left it blank. Now I got it, I'm moving to fix it up. It's a great explanation.