What do you want it to be?
1. You can have it replace your santilatatech.x10hosting.com domain.
2. You can keep santilatatech.x10hosting.com and "park" you co.cc domain so that the two url show the exact same site.
3. You can "addon" the co.cc domain, so that it points to a subdirectory of your site. That way you can have it show completely different content.
First you go to co.cc and use their account tools to adjust the nameservers for your co.cc domain to:
ns1.x10hosting.com
ns2.x10hosting.com
Then adjust your cPanel account by doing one of the three options above.