hi,
Add a reference to Telerik.Web.UI to your website and then register its assembly as:
<![CDATA[<%@ Register Assembly="Telerik.Web.UI" Namespace="Telerik.Web.UI" TagPrefix="telerik" %>]]>
and a combobox will look like
<telerik:radcombobox id="cmbRoles" skin="Forest" emptymessage="Apply" runat="server" xmlns:telerik="#unknown">
Width="130px">
</telerik:radcombobox>