In this article I will explain how to install AJAX Control Toolkit in ToolBox of Visual Studio 2008, 2010, 2012 and 2013.
AJAX Control Toolkit Library is not integrated in Visual Studio by default and hence I will explain a permanent way to add AJAX Control Toolkit controls to ToolBox in Visual Studio 2008, 2010, 2012 and 2013.
 
1. Download and Copy the AJAX Control Toolkit DLL
The very first task is to download the AJAX Control Toolkit DLL using the following download link.
Once downloaded, copy the AJAX Control Toolkit DLL to some fixed location in C drive or any drive of your choice.
Add AJAX Control Toolkit controls to ToolBox in Visual Studio 2008, 2010, 2012 and 2013
 
 
2. Add a New Tab in ToolBox
Now you need to open Visual Studio, right click in the ToolBox and click Add Tab.
Note: The screenshots shown are of Visual Studio 2013, but the steps are same for all versions of Visual Studio.
 
Add AJAX Control Toolkit controls to ToolBox in Visual Studio 2008, 2010, 2012 and 2013
 
 
3. Set a suitable name for the Tab
The Add Tab button will add a new Tab in the Toolbox and you need to set a suitable name to it.
Add AJAX Control Toolkit controls to ToolBox in Visual Studio 2008, 2010, 2012 and 2013
 
 
4. Choose Items for the ToolBox
Now inside the newly created Tab, you need to right click and click Choose Items…
Add AJAX Control Toolkit controls to ToolBox in Visual Studio 2008, 2010, 2012 and 2013
 
The above action will open the Choose Items Dialog in Visual Studio as shown below.
Add AJAX Control Toolkit controls to ToolBox in Visual Studio 2008, 2010, 2012 and 2013
 
You will need to click on Browse button to choose the AJAX Control Toolkit DLL from the folder it has been saved and click Open.
Add AJAX Control Toolkit controls to ToolBox in Visual Studio 2008, 2010, 2012 and 2013
 
If you get the following warning, simply click Yes.
Add AJAX Control Toolkit controls to ToolBox in Visual Studio 2008, 2010, 2012 and 2013
 
All the controls present in the AJAX Control Toolkit Library are now displayed in the Visual Studio Choose Items Dialog.
Add AJAX Control Toolkit controls to ToolBox in Visual Studio 2008, 2010, 2012 and 2013
 
This is the final step and now you need to click OK and the AJAX Control Toolkit controls will now be added to the Visual Studio ToolBox.
Add AJAX Control Toolkit controls to ToolBox in Visual Studio 2008, 2010, 2012 and 2013
 
Note: Make sure the AJAX Control Toolkit DLL is not deleted or moved from the Location it is saved as otherwise the ToolBox will not show the AJAX Control Toolkit controls.