XSLT Debugger is disabled in Visual Studio 2013 Integrated Shell

According to this link, the Visual Studio 2013 Integrated Shell contains an XSLT Debugger:

https://msdn.microsoft.com/en-us/library/bb129445.aspx?f=255&MSPPError=-2147217396

I installed this version clean on a machine that has no prior Visual Studio installs.

When I launch this version of Visual Studio, and open an xsl file for debugging, the 'Start XSLT Debugging' option is grayed out / disabled. 

How do I enable XSLT Debugging?

Thanks,

--gar