On your Toolbox Panel right click and choose "add tab".
Name the new tab "Ajax Control Toolkit"
Right click under the new tab where it says: "There are no usable controls..." and click "Choose Items".
A "Choose Toolbox Items" dialog appears.
Under ".NET Framework Controls" choose Browse.
Navigate to the directory where you have extracted the Ajax Control Toolkit.
Find SampleWebSite\Bin directory.
Choose AjaxControlToolkit.dll and click Open button.
After a while the dialog populates with the controls (they should be all selected by default).
Confirm with OK.