How to permenantly add AJAX Control Toolkit in ToolBox of Visual Studio

It is not working for you because you add it from the BIN folder of the project and hence when you open a different project it does not load.

A simple way is to copy the AJAX Control Toolkit DLL in some folder in your C drive.

Then add it to the Visual Studio ToolBox.

Now you will see it always provided the file opened currently is ASPX as the Toolkit Tab will not appear for .CS, .CSS or similar files where there's no use of it.