TinyMCE Fiddle

<link href="http://www.barelyfitz.com/projects/tabber/example.css" media="screen" type="text/css" rel="stylesheet">
<script type="text/javascript" language="javascript" src="http://www.barelyfitz.com/projects/tabber/tabber.js"></script>

<script type="text/javascript">
tinyMCE.init({
    // General options
    mode : "textareas", //tested also with exact mode
    theme : "advanced",
    content_css : "http://www.barelyfitz.com/projects/tabber/example.css", //**LOAD THE CSS STYLE FOR TABS
    plugins : "autolink,lists,spellchecker,pagebreak,style,layer,table,save,advhr,advimage,advlink,emotions,iespell,inlinepopups,insertdatetime,preview,media,searchreplace,print,contextmenu,paste,directionality,fullscreen,noneditable,visualchars,nonbreaking,xhtmlxtras,template,imagemanager,filemanager",
    
    // Theme options
    theme_advanced_buttons1 : "save,newdocument,|,bold,italic,underline,strikethrough,|,justifyleft,justifycenter,justifyright,justifyfull,|,styleselect,formatselect,fontselect,fontsizeselect",
    theme_advanced_buttons2 : "cut,copy,paste,pastetext,pasteword,|,search,replace,|,bullist,numlist,|,outdent,indent,blockquote,|,undo,redo,|,link,unlink,anchor,image,cleanup,help,code,|,insertdate,inserttime,preview,|,forecolor,backcolor",
    theme_advanced_resizing : true,
    width: "100%",
    height: "200",
    setup : function(ed) //**LOAD THE JAVASCRIPT INSIDE THE edit iframe
            {
                ed.onInit.add(function(ed, evt)
                {
                    alert("entered tiny onInit.add");
        	
    				//DOESNT LOAD THE JS
                    // Load a script from a specific URL using the global script loader
                    //tinymce.ScriptLoader.load(' http://www.barelyfitz.com/projects/tabber/tabber.js');
                    
                    //DOESNT LOAD THE JS
    				// Load a script using a unique instance of the script loader
                    //var scriptLoader = new tinymce.dom.ScriptLoader();
                    //scriptLoader.load('http://www.barelyfitz.com/projects/tabber/tabber.js');

                    // Load multiple scripts, WORKS
                    var scriptLoader = new tinymce.dom.ScriptLoader();
                    scriptLoader.add('http://www.barelyfitz.com/projects/tabber/tabber.js');
                    scriptLoader.loadQueue(function() { alert('All scripts are now loaded.'); });
                });
            }
});
</script>


<form method="post" action="dump.php">
    <textarea name="content">
    Code inside the tinymce edit iframe, the tabs are Not shown:<br />
    <div id="tabber1" class="tabber"> 
      <div class="tabbertab" title="FIRSTTAB">content firsttab</div>
      <div class="tabbertab" title="SECONDTAB">content secondtab</div> 
    </div>

    </textarea>
</form>

<br />

Code outside the tinymce edit iframe, the tabs are perfectly shown:<br />
<div id="tabber1" class="tabber"> 
  <div class="tabbertab" title="FIRSTTAB">content firsttab</div>
  <div class="tabbertab" title="SECONDTAB">content secondtab</div> 
</div>