Namespace Manager or XsltContext needed. This query has a prefix, variable, or user-defined function

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);