Unable to import form ExternalSchema

Platform : VC++, .NET 2005, MSXML6.0

Hi,

We are using the ISchemaModelGroupPtr and ISchemaItemPtr to read the
(xCBL v4.0 OrderManagement.xsd) XSD file's elements.
We are able to read all the elements referred to in the XSD except for the
DigitalSignature elements which are to be read from the "xmldsig-core-schema.xsd"
in a folder "ExternalSchema".

We get the following error:
[Error -2147467259 Unspecified error msxml6.dll {http://www.w3.org/2000/09/xmldsig#}SignedInfo is not found in the collection.]


"SignedInfo" is the Element that is to be read from the XSD.

Any pointers or suggestions as to what could be the problem?

Thanks and Regards
Asmita