Mystery: addIframe not showing... why?

Can anyone tell me why the IFRAME window is not showing when I press MyButton? addIframe does not seem to exist in "CKEDITOR.dialog".

<script type="text/javascript" src="ckeditor/ckeditor.js"></script>

<textarea id="editor1" name="editor1">Test</textarea>

<script>
CKEDITOR.plugins.add( 'iframedialog' ) ;
var editor1=CKEDITOR.replace( 'editor1');
editor1.config.toolbar =[['Cut','Copy','Paste','-','MyButton']] ;

editor1.on( 'pluginsLoaded', function( ev ) {
if ( !CKEDITOR.dialog.exists( 'myDialog' ) ) {
   CKEDITOR.dialog.addIframe( 'myDialog', 'title','http://www.google.be',500,400,function(){});
}
editor1.addCommand( 'myDialogCmd', new CKEDITOR.dialogCommand( 'myDialog' ) );
editor1.ui.addButton( 'MyButton', { label : 'My Dialog', command : 'myDialogCmd', icon: 'http://thereality.nl/images/icons/icon_upload.png' } );
});

</script>