Basically, if you want to add a rich text area to your dialogs, you can use the addRichtextField method. However, in case you want to add the rich text editor manually, you first need to use the addTextAreaField method and some additional Javascripts as shown below:
<%
String[] fieldDescription = ["jcrPath=/node/exo:description"] ;
uicomponent.addTextAreaField("description", fieldDescription)
%>
<script>
var instances = CKEDITOR.instances['description'];
if (instances) instances.destroy(true);
CKEDITOR.replace('description', {
toolbar : 'CompleteWCM',
uiColor : '#9AB8F3'
});
</script>
