To install the Ajax Control Toolkit,
first download the zip file from www.asp.net/ajaxlibrary/download.ashx.
Unzip the files into any directory.
Open your Visual Web Developer and right click in the Toolbox area. Select "Add Tab."
Name the tab with something like "AjaxTooKit."
Right click the new tab area, and select "Choose Items..."
This will bring up a dialog box to "Choose Toolbox Items." Select the "Browse" button.
Find and select to open the "AjaxControlToolkit.dll" file.
The Choose Toolbox Items dialog will appear, this time with the Ajax Tookit controls selected. Select the "OK" button.
Open or create a web form within your project. Now when you open your toolbox, you will see all of the Ajax Control Toolkit controls, ready for use within your applications.