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http://dhtmlx.com/docs/products/kb/index.php?s=normal&q=7304

Now i included it manualy, but same error. Here my code:

<link rel="STYLESHEET" type="text/css" href="../script/treetool/dhtmlxtree.css">
<script  src="../script/treetool/dhtmlxcommon.js"></script>
<script  src="../script/treetool/dhtmlxtree.js"></script>
<script  src="../script/treetool/ext/dhtmlxtree_start.js"></script>
<script  src="../script/treetool/ext/dhtmlxtree_wz.js"></script>

[...]

var lastDelId;
user_tree=new dhtmlXTreeObject("inhalt_treemenu","100%","",0);
user_tree.setImagePath("../script/treetool/imgs/");user_tree.setDragBehavior("complex");
user_tree.enableDragAndDrop(1);
user_tree.setDragHandler(myDragHandler);user_tree.setOnDblClickHandler(onNodeSelect);
user_tree.attachEvent("onOpenEnd",function(){ 
window.setTimeout(function(){ // need to wait while open tread will be closed
user_tree.saveOpenStates(); 
},1);
});
user_tree.loadXML("xml.php",function(){ user_tree.loadOpenStates(); }); 

do i have zu configure some files to include dhtmlxtree_wz.js correctly?

While you are using
  dhtmlxtree_wz.js
Correct file name is 
  dhtmlxtree_xw.js 

If necessary - you can contact us directly at support@dhtlmx.com - we will provide a separate sample for functionality in question.

ok, this was an easy mistake *shame on me*
Now it works!

but..
in FF Error Console i get an new Error:  "tree is not defined"  source: (..)/dhtmlxtree_xw.js  line: 17

Thanks for your help!

It seems that you are using not latest codebase, such error was confirmed and fixed for one of earlier build.
Please try to use updated file ( sent by email ) instead of original one 

now all is ok, thank you!