Change the root and add namespace to your selects

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" versi
+on="1.0">
<xsl:output method="xml" indent="yes"/>
<xsl:template match="/">
<Data xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<xsl:for-each select="xsd:File/xsd:DataDescrip">
<xsl:element name="Details">
  <xsl:element name="ReferenceID">
    <xsl:value-of select="xsd:Reference"/>
  </xsl:element>
</xsl:element>
</xsl:for-each>
</Data>
</xsl:template>
</xsl:stylesheet>
[download]

gives

<?xml version="1.0"?>
<Data xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  

    <ReferenceID>702154</ReferenceID>
  
</Data>