This is my XSLT:
<?xml version="1.0"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:msxsl="urn:schemas-microsoft-com:xslt" xmlns="urn:schemas-microsoft-com:office:spreadsheet" xmlns:o="urn:schemas-microsoft-com:office:office" xmlns:x="urn:schemas-microsoft-com:office:excel" xmlns:ss="urn:schemas-microsoft-com:office:spreadsheet">
<xsl:output method="xml" indent="yes"/>
<xsl:template match="/">
<xsl:processing-instruction name="mso-application">
<xsl:text>progid="Excel.Sheet"</xsl:text>
</xsl:processing-instruction>
<Workbook>
<Worksheet ss:Name="Exported Sheet">
<Table>
<Row>
<Cell ss:mergeAcross="2">
<Data ss:Type="String">Product Group</Data>
</Cell>
</Row>
<Row>
<Cell>
<Data ss:Type="String">MSCI</Data>
</Cell>
<Cell>
<Data ss:Type="String">Rel-</Data>
</Cell>
</Row>
</Table>
</Worksheet>
</Workbook>
</xsl:template>
</xsl:stylesheet>

Any idea why my mergeAcross doesn't work?

Thanks!