Here i have explained how to Install AjaxControlToolkit in Asp.Net.
First of all Download
'AjaxControlToolkit-Framework3.5SP1.zip' file from the ASP.NET AJAX Control Toolkit Releases page.
Step 1: Open Toolbox > Right click on the Toolbox > Click on Add Tab
Step 2: Give name to the Tab
Step 3: Click on Choose Items
Step 4: Select ".NET Framework Components" Tab and Click on "Browse" Button
Step 5: Select "AjaxControlToolkit.dll" file and click on "Open" Button
Step 6: Click on "Ok" Button
Done: now you can see installed AJAX Control Toolkit here