You have to add xsl
namespace to XmlNamespaceManager
:
var document = new XmlDocument();
document.Load(...);
var nsmgr = new XmlNamespaceManager(document.NameTable);
nsmgr.AddNamespace("xsl", "http://www.w3.org/1999/XSL/Transform");
var nl = document.SelectNodes("//xsl:import/@href", nsmgr);