Add an XML attribute on a specific location of a specific node:

//Look for node MyNode

XmlNode node = doc.SelectSingleNode("//MyNode");

//Create empty attribute

XmlAttribute attribute = doc.CreateAttribute("MyAttrib2");

//Remove any existing attribute with that name just in case

node.Attributes.Remove(attribute);

//Set the value of the attribute

attribute.Value = "1";

//Add the attribute to the node

XmlAttribute attributeBefore = node.Attributes["MyAttrib1"];

node.Attributes.InsertAfter(attribute, attributeBefore);