You can check the syntax that you used on this documentation page: About advanced XML If you have followed these steps exactly, and it doesn't work, you should file a case with Splunk Support.
However, while you can still use advanced XML in Splunk 6.x, you might consider converting your simple XML into HTML instead. Once you do so, you can use a variety of libraries and extensions to customize your dashboards. Here is a link to the documentation on converting a dashboard to HTML. But you may find it more useful to download the Splunk 6.x Dashboard Examples App and look at the examples and the code.