The 'content' property of panel has been defined in easyui, you must to select another name such as 'content2'.
To display the 'content2' inside a tab panel when opened, try this:
<div id="tt" class="easyui-tabs" style="width:700px;height:250px">
<div title="About" style="padding:10px" data-options="content2:'#myDIV'"> </div>
<div title="My Documents" style="padding:10px"> </div>
<div title="Help" data-options="iconCls:'icon-help',closable:true" style="padding:10px"> </div>
</div>
<div id="myDIV">
My content
</div>
<script>
$.fn.panel.defaults.onOpen = function(){
var opts = $(this).panel('options');
if (opts.content2){
$(opts.content2).appendTo(this);
}
};
</script>