Can't get ajax controls to compile in visual studio 2012

Problems getting ajaxcontroltoolkit to work in visual studio.

I have an old asp.net website (not a projectc) that used ajax.  I just have the asp.net files.  So I did the following.  I created a bin folder, and unzipped the ajaxcontroltoolkit into it.  Then I used the Nuget Package Manager in Visual Studio to install AjaxControlToolkit.  

Then I compiled it.  I got this message:

1) Error 1 Unknown server tag 'asp:scriptmanager'. C:\shmuelKatz\Advanced2.aspx 36

and this message:

2) 

Error 11 Reference required to assembly 'System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' containing the base class 'System.Web.UI.ExtenderControl'. Add one to your project. 

So I went to visual studio, and right-clicked on an empty tab, and picked "choose items" and then a dialog box popped up filled with COM objects and NET objects.  But none of them started with the word 'ajax'.

I'm puzzled.  In case there is something wrong with my web.config, here it is:

<?xml version="1.0" encoding="utf-8"?><!--
    Note: As an alternative to hand editing this file you can use the
    web admin tool to configure settings for your application. Use
    the Website->Asp.Net Configuration option in Visual Studio.
    A full list of settings and comments can be found in
    machine.config.comments usually located in
    \Windows\Microsoft.Net\Framework\v2.x\Config
--><configuration><system.webServer><httpErrors errorMode="Detailed" /><asp scriptErrorSentToBrowser="true" /></system.webServer><system.web><customErrors mode="Off" /><pages><controls><add tagPrefix="ajaxToolkit" assembly="AjaxControlToolkit" namespace="AjaxControlToolkit" /></controls></pages></system.web></configuration>

Any help is much appreciated.

-- Gid