1- Download mootools 1.2 package // Bereits im Paket enthalten
   -> Go here : http://mootools.net/download/
   -> Download MooTools 1.2.x with "YUI Compressor" compression type

2- Rename the downloaded file to mootools-1.2.x.js // Auch schon geschehen

3- Send it on your server on wwwroot/theme/<your theme>/js/

4- Edit your theme.html (wwwroot/theme/<your theme>/theme.html)

   -> Add this value to the head part :
   <script src="<{xoImgUrl js/mootools-1.2.x.js}>" type="text/javascript"></script>

5- Check if this value <{$xoops_module_header}> is present in the head of theme.html and AFTER the mootools include (point 4)

6- Empty your wwwroot/xoops_data/caches/smarty_compile/ except the index.html file

Enjoy