Inserting text to CKEditor cursor location using asp.net (C#)

I'm using CKEditor.Net in a asp.net (C#) Web Application Project. I need to insert some text into CKEditor cursor location, when a user clicks a server side button.

This is my code,

HTML

<%@ Page Title="" Language="C#" MasterPageFile="~/Site.Master" AutoEventWireup="true"    CodeBehind="Test.aspx.cs" Inherits="eLetters.Test" %>
<%@ Register Assembly="CKEditor.NET" Namespace="CKEditor.NET" TagPrefix="CKEditor" %>

<asp:Content ID="Content1" ContentPlaceHolderID="HeadContent" runat="server">
    <script type="text/javascript">
        function insertText(text) {
            CKEDITOR.instances.insertText('text');
        }
    </script>
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="FeaturedContent" 
  runat="server">
</asp:Content>
<asp:Content ID="Content3" ContentPlaceHolderID="MainContent" runat="server">
    <CKEditor:CKEditorControl ID="CKEditor1" BasePath="/ckeditor/"   
       runat="server"></CKEditor:CKEditorControl>
</asp:Content>

    protected void Button1_Click(object sender, EventArgs e)
    {
        Page.ClientScript.RegisterStartupScript(GetType(), "MyKey",  
          "insertText('data');", true);
    }

CKEditor works fine, but I get this error when I click the button (to insert some text to curser locatin),

Error: 'CKEDITOR' is undefined

How to fix this?

Thanks in advance...