Ajax Control Toolkit Example: Using RoundedCornersExtender

Hi

I copy/paste from

http://asp-net-example.blogspot.co.uk/2009/11/ajax-roundedcornersextender-how-to-use.html

but it gives me following error(I already add najaxToolkit.dll in bin folder)

How to set this in web.config file

Thanking you

Anil Srivastava