1) Download the latest version from CodePlex for AJAX.
2) Extract to a location of your choice
3) Add the AjaxControlToolkit.dll & AjaxControlToolkit.pdb to you Web layer project /bin folder
4) Create a new Tab in your Toolbox and point to the new AjaxControlToolkit.dll
5) Create a reference to your AjaxControlToolkit.dll
6) Finally ensure the line in your Web.config is that same version as your dll assembly -