1 In co.cc go to -->manage domains
2 Click "Setup"
3 Choose "Nameservers" by clicking on dot
4 There enter :ns1.x10hosting.com
ns2.x10hosting.com
as your nameservers.Save settings.By now half part of it is over and co.cc sends the requests coming for your domain to x10 servers.
Now for X10 servers to know that only your account holds it and that it is to transfer that request to your site,
Do the following.
1. Login to cpanel ..
2. Click addon domains( alternatively you can choose subdomain aswell)
3. Then set up the domain and choose the folder to which your domain should be targetted.
Save..
The following is a video that will help you better
http://www.youtube.com/watch?v=unA1OVVcxFo
Revert back if you still have a problem.