var reportXSLTFile: <Error class: unknown class>

The path to the XSLT transformation file relative to the Build Checkout Directory or absolute on the agent machine. You can use the path to the detected FxCop on the target machine (i.e. %system.FxCopRoot%/Xml/FxCopReport.xsl). When the Report XSLT file option is set, the build runner will apply an XSLT transform to FxCop XML output and display the resulting HTML in a new FxCop tab on the build results page.