// (c) Copyright Microsoft Corporation.
// This source is subject to the Microsoft Public License.
// See http://www.microsoft.com/opensource/licenses.mspx#Ms-PL.
// All other rights reserved.

You must go to 
and download ajaxcontroltoolkit 
then Copy AjaxToolKit Like Picture 
and paste them to Bin Folder  

Right Click on solution choose Add Reference 
in the browse tab double click on the Bin Folder, 
and double click on ajaxcontroltoolkit 
then on the Build Menue > click Rebuild.
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