<p>I added the ModalPopupExtender to an existing ASP.NET application. At runtime I get the following error</p>

<p>Compilation Error 
Description: An error occurred during the compilation of a resource required to service this request. Please review the following specific error details and modify your source code appropriately. </p>

<p>Compiler Error Message: BC32206: Indirect reference is being made to assembly System.Web.Extensions version 3.5.0.0, which contains 'AjaxControlToolkit.ModalPopupExtender'. This Project references a prior version of System.Web.Extensions version 1.0.61025.0. To use 'AjaxControlToolkit.ModalPopupExtender', you must replace the reference to System.Web.Extensions with version 3.5.0.0 or higher.</p>

<p>Source Error:</p>

<p>Line 29:         <br />
Line 30:<br>
Line 31:         </p>

<p>How do I fix this. </p>

You're trying to compile a .NET 3.5 application with a reference to a .NET 1.1 module.  Make sure you have the .NET 3.5 framework installed, and if you do, make sure you're trying to build it in .NET 3.5.
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