The PanelBar does not offer horizontal scrolling per default. The reason for that is that horizontal scrolling will interfere with the vertical scrolling.
You could, however, set scrolling for the RadPane. And since the RadPane is a bit beyond my expertise, I am forwarding the ticket to the devs who are taking care for that control.
In the mean time, if you have any other questions about the PanelBar, do not hesitate to ask them.
All the best,
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed