//*/@*[namespace-uri()='http://ns.adobe.com/AdobeInDesign/4.0/' and local-name()='pstyle']
also you need to add this after you declare the root element and before you evaluate the expression
if( !xml.xmlAttributes.itemByName("xmlns:xml").isValid )
xml.xmlAttributes.add( "xmlns:xml", "http://www.w3.org/XML/1998/namespace" );
if( !xml.xmlAttributes.itemByName("xmlns:aid").isValid )
xml.xmlAttributes.add( "xmlns:aid", "http://ns.adobe.com/Adobe InDesign/4.0/" );
if( !xml.xmlAttributes.itemByName("xmlns:aid5").isValid )
xml.xmlAttributes.add( "xmlns:aid5", "http://ns.adobe.com/Adobe InDesign/5.0/" );