Using .Net Framework Source debugging

I'm getting a null-object exception deep within XsltCompiledTransform, and I'd like to use the .Net Framework Source debugging feature to see what isn't being initialized, but even after following the directions for enabling it, I can't figure out how to step into the .Transform() method and start spelunking.

The tutorials I've found online say you can load the symbols for the framework library by using the Call Stack window while debugging, but this isn't useful when the callstack has yet to contain the library you want to debug. I've also tried loading symbols from the Modules window, but Step-Into still doesn't work.

Is the System.Xml.Xslt namespace source even available to step into when debugging?

From stackoverflow C. Lawrence Wenham

• If you have trouble fetching the sources on the fly, you might want to try out .NET Mass Downloader

  As it looks, the sources for XsltCompiledTransform are included.

  From Christian.K