关于dhtmlXTreeObject is not defined的问题

以下是我tree.html的内容 为什么浏览器访问tree.html时会出现dhtmlXTreeObject is not defined的警告 界面什么都不显示：我的路径是写真确的<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
  <head>
    <title>tree.html</title>

    <meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
    <meta http-equiv="description" content="this is my page">
    <meta http-equiv="content-type" content="text/html; charset=UTF-8">
    <link rel="stylesheet" type="text/css" href="/dhtmlxTree/dhtmlxTree/codebase/dhtmlxtree.css">
   <script type="text/javascript" src="/dhtmlxTree/dhtmlxTree/codebase/dhtmlxcommon.js"></script>
   <script type="text/javascript"  src="/dhtmlxTree/dhtmlxTree/codebase/dhtmlxtree.js"></script>  </head>
  <body>
<div id="box_tree"></div>   
   <script > 
     tree=new dhtmlXTreeObject('box_tree',"100%","100%",0);  
     tree.setXMLAutoLoading("/xml/node.xml");
     tree.loadXML("/xml/node.xml");            
  </script>   
  </body>
</html>