first you have download ajaxcontroltoolkit from here
http://ajaxcontroltoolkit.codeplex.com/releases/view/109918[
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then
In Your Visual Studio
Go to Toolbox->
then Right Clik on Any Tab and--> Click to Add Tab
give name AjaxControlToolkit
then Right Clik and Choose Item-->
then you see the Browse option
fetch the .DLL file which is in your AjaxControlToolkit Folder
And Click Ok .
and then add this line client side
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="ajaxToolkit" %>
then visit this link
http://www.asp.net/ajaxLibrary/AjaxControlToolkitSampleSite/BalloonPopup/BalloonPopupExtender.aspx[
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