Install the ASP.NET AJAX Control Toolkit
Download the ZIP file AjaxControlToolkit-Framework3.5SP1-DllOnly.zip from the ASP.NET AJAX Control Toolkit Releases page of the CodePlex web site.
Copy the contents of this zip file directly into the bin directory of your web site.
Update web.config
Put this in your web.config under the <controls> section:
<?xml version="1.0"?>
<configuration>
...
<system.web>
...
<pages>
...
<controls>
...
<add tagPrefix="ajaxtoolkit"
namespace="AjaxControlToolkit"
assembly="AjaxControlToolKit"/>
</controls>
</pages>
...
</system.web>
...
</configuration>
Setup Visual Studio
Right-click on the Toolbox and select "Add Tab", and add a tab called "AJAX Control Toolkit"
Inside that tab, right-click on the Toolbox and select "Choose Items..."
When the "Choose Toolbox Items" dialog appears, click the "Browse..." button. Navigate to your project's "bin" folder. Inside that folder, select "AjaxControlToolkit.dll" and click OK. Click OK again to close the Choose Items Dialog.
You can now use the controls in your web sites!