DNS functionality on the CSS is pretty limited. The CSS can only resolve 'a' record requests for a particular sub-domian.
To make this work you'll need to configure NS records on higher level DNS servers using the circuit VLAN address of the CSS. From there, you type 'add DNS' and the domain name on the content rule for which you would like to resolve the request.
Hope that helps answer your question.
Cheers,
Perry.