The example I pointed doesn't use the RadBusyIndicator control, but emulates it using the RadProgressBar control. The example is not meant to display BusyContent, there is some work to be done to implement this functionality.
Generally you need to add one more attached property to the BusyIndicatorExtensions class, holding the BusyContent. After that you need to pass its value to the BusyIndicatorWindow when opening it in the constructor of the BusyIndicatorExtensions class. As the final step you need to present the BusyContent in the BusyIndicatorWindow. I would suggest you to use a ContentPresenter for this purpose.
Hope this helps.
the Telerik team