First, see the general instructions at INSTALL.html.
To assign xml VFS to *.xml files (ignore case) and to XML file type add the following to mc.ext:
regex/\.([xX][mM][lL])$ Open=%cd %p#xml type/^XML *(1\.0 *)?document text Open=%cd %p#xml
Enable file types guessing.
Edit mc.ext; the syntax is slightly different:
regex/\.([xX][mM][lL])$ Include=xml type/^XML *(1\.0 *)?document text Include=xml include/xml Open=%cd %p/xml://
Enable file types guessing.