Try the following simple code :
public String s = Server.mappath( "\\Titles.xml");
XmlTextWriter writer = new XmlTextWriter(s, null);
writer.WriteStartElement("menu");
writer.WriteStartElement("buttons");
writer.WriteStartElement("button");
writer.WriteElementString("label", textBox1.Text);
writer.WriteEndElement();
writer.WriteEndElement();
writer.WriteEndElement();
writer.Close();
Where s is the name of your xml file that is you have.