Question : What is the process for getting the proper dll to use with 'example' code?  system.web.extensions.dll dll question

I am learning about .NET & AJAX
I have a error shown at the bottom of this note.
I tried to replace the System.Web.Extensions, "Version=1.0.61025.0" with the published number of:  4.0.30319.1  but still get a error.

What is the process for getting the proper dll to use with 'example' code?

Following a Microsoft example, i get stuck with the error:
Parser Error Message: Could not load file or assembly 'System.Web.Extensions, Version=4.0.30319.1, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. This assembly is built by a runtime newer than the currently loaded runtime and cannot be loaded. (C:\jer\ListSearch\web.config line 41)

Answer : What is the process for getting the proper dll to use with 'example' code?  system.web.extensions.dll dll question

Check the AJAX control toolkit sample
http://www.asp.net/ajax/ajaxcontroltoolkit/samples/

Download it and use it.
You will get the working example of the system.web.extensions