If you are using asp.net 3.5 you need to use a different version of the Ajax Control toolkit than you would use if you are not using asp.net 3.5.

http://www.codeplex.com/AjaxControlToolkit

So if you are using asp.net 3.5 first upgrade to the new toolkit (put the new .dll in your bin folder in your DNN website).

Perhaps you don't have Ajax installed on your site? You must have Ajax working before trying to use the toolkit. Also even if you have AJax working on your site, you have to make modifications to the web.config.

What you can do is create a dev dnn site from scratch. Open it in Visual Studio 2008. This will cause all the changes to be added to the web.config. Then when you add the AJax Control toolkit (the 3.5 version) it will work.

Then you can compare the web.config that works to the one that doesn't.