I think I’ve posted on this before and this is really just an update that I can refer to for future use. Basically to debug a map you right mouse on the btm file within Visual Studio and select validate map. Then ctril+click the link to the generated xsl file in the output window. Right mouse on the xsl and select view source. You can now put break points in the xsl. Now from the visual studio XML menu select the Debug XSLT item. You’ll be prompted to supply an instance document and then visual studio will launch into debugging showing the output being built as you step through the XSLT.
But what if you use functoids! Out of the box XSLT debugging doesn’t provide a facility for loading extension xml which is pretty ordinary really. Never fear the BizTalk community is here. Many moons ago Jon Flanders posted this gem, http://www.masteringbiztalk.com/blogs/jon/PermaLink,guid,bc5e05f0-659f-4ace-a6fe-d864cdf1fbe2.aspx.
Something worth mentioning is this technique is pretty much obsolete nowadays. In Visual Studio 2008/BizTalk Server 2009 we now have the debug map option alongside validate and test map. This automates all of the above for us. Still I find myself doing a LOT of work in BizTalk Server 2006 R2 and expect to for the next couple of years to come.