The Advanced Mode (embed code) textbox in the Insert External Video dialog is always readonly and you cannot put the code in it.

In the meantime you can use the following workaround that will access the textbox and remove the readonly attribute:

	<telerik:RadEditor ID="RadEditor1" runat="server" OnClientCommandExecuted="OnClientCommandExecuted">
		<Tools>
			<telerik:EditorToolGroup>
				<telerik:EditorTool Name="InsertExternalVideo" />
			</telerik:EditorToolGroup>
		</Tools>
	</telerik:RadEditor>
	<script type="text/javascript">
		function OnClientCommandExecuted(sender, args)
		{
			if (args.get_commandName() == "InsertExternalVideo")
			{
				var extVideoDialog = sender.get_dialogOpener()._dialogContainers["InsertExternalVideo"];
				if(extVideoDialog){
					extVideoDialog.add_pageLoad(editTextbox);
				}
			}
		}

		function editTextbox(sender, args)
		{
			sender.get_contentFrame().contentWindow.document.getElementById("embedCode").removeAttribute("readonly");
			sender.remove_pageLoad(editTextbox);
		}
	</script>