Use Telerik Ajax Controls in Custom-built SharePoint 2010 Web Part

1. First you need to install Telerik.Web.UI.dll and Telrik.Web.Design.dll in the GAC. The simplest way is to drag & drop the file to ‘windows\assembly’ folder. 
2. Next you should tell SharePoint that the controls from the Telerik.Web.UI.dll is safe to use.
   To do so you should add a few <SafeControl> entries within the <SafeControls> section in your web.config:
   <SafeControl Assembly="Telerik.Web.UI, Version=2010.2.713.35, Culture=neutral, PublicKeyToken=121fae78165ba3d4" Namespace="Telerik.Web.UI" TypeName="*" Safe="True" />
   
3. Register ‘Telerik’ prefix
   When drag Telerik Ajax component onto user control (.ascx), Visual Studio will automatically add the following to the top of ascx file:
   <%@ Register assembly=”Telerik.Web.UI, Version=2010.2.713.35, Culture=neutral, PublicKeyToken=121fae78165ba3d4″ namespace=”Telerik.Web.UI” tagprefix=”telerik” %>