#setNodeValue("/my:myFields/my:Form/my:LeaveType, "Annual");
#setNodeValue(XPath, ValueString);
public void setNodeValue(string path, string value)
    {
                XPathNavigator root, infopathNode;
                root = this.MainDataSource.CreateNavigator();
                infopathNode = null;
                infopathNode = root.SelectSingleNode(path, this.NamespaceManager);
                infopathNode.SetValue(value);
    }