Question: how to add ajaxcontroltoolkit.dll in visual studio?
Answer: 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. ... 
Add a New Tab in ToolBox. Now you need to open Visual Studio, right click in the ToolBox and click Add Tab.
Set a suitable name for the Tab. ... 
Choose Items for the ToolBox.