When you are looking for a node which has an attribute "name" set to a particular value, you need to use different syntax.
You're looking for something like:
XmlNode node = doc.SelectSingleNode("/whois-resources/objects/object/attributes/attribute[@name=\"descr\"]");
XmlAttribute attrib = node.Attributes["value"];
MessageBox.Show(attrib.Value);
This will select your second node example, get the value of the value attribute, and display it.