I have used the ajaxtoolkit with .NET 4 and Visual Studio 2010, and I was able to add a new tab to the toolbox and add the ajaxtoolkit controls to the toolbox. How
I just installed it in Visual Studio 2013. Here's a summary of the process:
Click on solution and Manage Nuget Packages
Filter for AJAX and select the Ajax Control Toolkit
After NuGet installs this, it won't be in the toolbox!
Create a new toolbox group for AJAX CONTROL TOOLKIT
Add Items
Browse
Navigate to the toolkit URL. Project is Webapplication1 in the c:\temp folder so the toolkit was in: C:\temp\WebApplication1\packages\AjaxControlToolkit.7.1213\lib\45 (because I am using framework 4.5)