hey greg, I found out that custom extension xml works only with custom xslt path.
i tried out the xslt file, which was generated by validate map, in custom xslt path. And there is an attribute field which looks like this (in the *.btm file):
<CustomXSLT XsltPath=”..\..\..\..\..\AppData\Local\Temp\_MapData\IWMappingOpenImmo_to_IW\IWMappingOI_to_IW.xsl” ExtObjXmlPath=”.\IWMappingOI_to_IW_extxml.xml” />
So I believe that the custom extension xml file works only if you work with your own xslt file. But I dont. I am working with the xslt file generated by VS2010.