ODS TAGSETS.EXCELXP creates Excel 2002/2003 Spreadsheet Markup Language XML files -- this conforms to an XML standard that existed -before- the newest Office 2007 XML standard.
You cannot create .XLSX files using ODS TAGSETS.EXCELXP. An XLSX file is actually a zip archive of a set of related files -- all of which taken together make up a workbook/worksheet definition in Office 2007 XML. This is a different standard than the 2003 standard.
The only way to create XLSX files for Office 2007 is to use the SAS Excel Libname engine (or other methods) as described in this Tech Support note: http://support.sas.com/kb/32/455.html