You need to declare the namespace in your XSLT, and use it in XPath expressions. E.g.:
<xsl:stylesheet ... xmlns:my="http://www.mysite.com">
<xsl:template match="/my:MyRoot"> ... </xsl:template>
</xsl:stylesheet>
Note that you must provide some prefix if you want to refer to elements from that namespace in XPath. While you can just do xmlns="..."
without the prefix, and it will work for literal result elements, it won’t work for XPath – in XPath, an unprefixed name is always considered to be in namespace with blank URI, regardless of any xmlns="..."
in scope.