1) you have to make the index.html file so the file will be loaded automatically by server when you go to your primary domain
2) For own domain
First you have to register the co.cc domain at www.co.cc
then follow http://x10hosting.com/wiki/How_to_Use_Your_Own_Domain
The 500 error may be caused by the files and folder permissions, Check them whether they set the permissions to 755 and 644
The 777 permissions causes the issues.
It is loading fine for me. clear your browser cache, Flush your DNS and then try again.
One more thing If you set the nameservers today then the DNS propagation may take upto 48hours.