Read lot of threads on this which are having same problem ,some of them got solved none of them posted how did they succeded on those.
i am getting error like Name "ds:Signature" is incorrect, when i tried to inport the .dtd file as external definition,so I did try converting dtd file to XSD ,it is creating the 3 xsd files and none of them are valid each one of them are error.But however the DTD file is valid .
Using Invoicedetail DTD file for my scenario.
you can download the file from http://cxml.org
Please let me knw on this.