Marc_s's answer is actually correct. You should pay attention to the XML namespaces. However, his sample code will not work directly for your example. Here is the full sample that works with the XML you gave (although I had to clear it ... it was missing the closing tag for attributes).
string xmlData =
@"<?xml version='1.0' encoding='utf-8'?>
<StructureResponse
xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance'
xmlns:xsd='http://www.w3.org/2001/XMLSchema'
xmlns='http://nts-de-osm1-pxc/webservices/'>
<consolidatedItems xsi:nil='true' xmlns='http://systinet.com/wsdl/com/osm/webservices/service/' />
<rootItem xsi:type='Part' xmlns='http://systinet.com/wsdl/com/osm/webservices/service/'>
<attributes>
<Attribute>
<dataDictionary xsi:nil='true' />
<dataType>string</dataType>
<displayName>IDENT_NR</displayName>
<key>true</key>
<name>IDENT_NR</name>
<searchable>true</searchable>
<userAttribute>true</userAttribute>
<value>9662744</value>
</Attribute>
<Attribute>
<dataDictionary xsi:nil='true' />
<dataType>string</dataType>
<displayName>AI</displayName>
<key>true</key>
<name>AI</name>
<searchable>true</searchable>
<userAttribute>true</userAttribute>
<value>00</value>
</Attribute>
</attributes>
</rootItem>
</StructureResponse>";
XmlDocument document = new XmlDocument();
XmlNamespaceManager namespaceManager = new XmlNamespaceManager(document.NameTable);
namespaceManager.AddNamespace("a", "http://nts-de-osm1-pxc/webservices/");
namespaceManager.AddNamespace("b", "http://systinet.com/wsdl/com/osm/webservices/service/");
document.LoadXml(xmlData);
XmlNodeList nodes = document.SelectNodes("/a:StructureResponse/b:rootItem/b:attributes/b:Attribute", namespaceManager);
// 'nodes' contains 2 items now, as expected
XML. skimming " XML" .