By default the XML file creates a reference to an XSLT file "log.xsl" in the current directory; look in ANT_HOME/etc for one of these. You can set the property ant.XmlLogger.stylesheet.uri to provide a uri to a style sheet. this can be a relative or absolute file path, or an http URL. If you set the property to the empty string, "", no XSLT transform is declared at all.
Not quite sure what your question is. By default it is looking for log.xsl in current directory which doesn't exist unless you put one there. There's one in ANT_HOME/etc as the quote above indicates. If you want to point to one somewhere other than current directory then set the property as any other property, two of the common ways shown here: