<?xml version="1.0" encoding="utf-8"?>
<root>
  <left>
  </left>
  <center>
    <div1>
        <![CDATA[ <div id="div1"><div> ]]>
    </div1>
  </center>
  <right>
  </right>
</root>

public static void XmlInsertHtmlCode(string xmlPath, string MainNode, string Element, string Content)
        {
            XmlDocument objXmlDoc = new XmlDocument();
            objXmlDoc.Load(xmlPath);
            XmlNode objNode = objXmlDoc.SelectSingleNode(MainNode);
            XmlElement objElement = objXmlDoc.CreateElement(Element);
            objElement.InnerXml = Content;
            objNode.AppendChild(objElement);
            objXmlDoc.Save(xmlPath);
        }

XmlInsertHtmlCode("xxx.xml","root/center","div1","<div id="div1"><div>");