On the Telkirk sample they have this in the aspx page:
<%@ Register TagPrefix="telerik" Namespace="Telerik.Web.UI" Assembly="Telerik.Web.UI" %>
with the control defined with the tag:
<telerik:RadPanelBar ...
and the following using statement in the C# code-behind file:
using Telerik.Web.UI;
Check out the sample here, maybe it'll help if what I just said didn't fix it.
http://demos.telerik.com/aspnet-ajax/panelbar/examples/functionality/firstlook/defaultcs.aspx
-w00te