I buy all my domains at godaddy.com and then I set the NAME SERVERS for my domains on godaddy.com to point to cirtexhosting. These settings were given to you in an email when you first signed up with cirtex. Check for that email. It will say something like NXX.CIRTEXTHOSTING.COM and NXX.CIRTEXHOSTING.COM (where "XX" is a number).

If you don't know what they are, contact cirtex and ask, they'll tell you.

You then need to go to United Domains where you bought your domains, and in your account you should be able to access the "Set Name Servers" for the domains. It's usually under "manage domains" and then "set name servers" (most domain cpanels have this kind of thing so poke around till you find it). Make sure you don't have the domains forwarded or parked or anything else and that you have the cirtex name servers in there. If you still need help setting the name servers, contact the folks at united domains and ask them for help doing that. Make sure you have the cirtex name server information available to give them.