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
To install Ajax Control Toolkit, run the following command in the Package Manager Console
PM> Install-Package AjaxControlToolkit
this is install the Ajaxtoolkit in your current project..
The current version of the AJAX Control Toolkit does everything to install itself into the VS toolbox. First exit out of VS (it will tell you to exit if you don't) then click the Download button in AJAX Control Toolkit.
Slight sort cut for finding the dll folder, for example step 7 in the instruction set given by user3546149.
(Good job, I would up vote your answer but no reputation).
AjaxControlTookkit should show under References for the project. Right click it and select Properties. The Path property is the path to the AjaxControlToolkit.dll. Copy that Path property.Then for step 7 paste the Path property directly in the search box. Saves eye strain and clickity click oops syndrome.
(Works for VS 2013 also, maybe others).
I just installed it in Visual Studio 2013. Here's a summary of the process:
It's located in the packages folder created by Visual Studio 2012 after you use NuGet Package Manager to download it.
Create a tab in Visual Studio (you can name it Ajax Toolkit).
Right click on it and select: Choose Items..
Click Browse and navigate to the packages folder that Visual Studio 2012 creates with your source. Something like this: ..\packages\AjaxControlToolkit.7.0123\lib\40\AjaxControlToolkit.dll
Select AjaxControlToolKit.