Cannot build Aspose Examples

I have downloaded the Aspose examples for PDF and am trying to build the VS 2010 solution.  In the "DigitallySignPDFFile" project, I am getting several error messages of the form: The type or namespace name 'PKCS7' could not be found (are you missing a using directive or an assembly reference?)

I also get this for 'DocMDPSignature'. 

Am I missing something? 

Thanks

Steve