Although there is a default script manager in DNN 4.5 or later, you still may get the following exception:

DotNetNuke.Services.Exceptions.PageLoadException: The control with ID 'RadMenu1' requires a ScriptManager on the page. The ScriptManager must appear before any controls that need it.

This error may occur if you are using .net 2 assembly in an asp.net 3.5 web site or vice versa. Please check if this is the case. Please, have in mind that we ship the SkinObjects with Telerik.Web.UI assembly built against .net 2.0. If you have configured your dnn for .net 3.5, you need to use the .net 3.5 version of the Telerik.Web.UI assembly which can be found in the bin35 folder of your installation of the telerik asp.net ajax suit.