Hi,
I had same problem and take me a few hours (spend few hours one day, no luch, dare try few hours again one week later, solved) to find out problem and resolve it.
The reference System.Web.Extensions you used in your project/web site might be 3.6.0.0 , and the AjaxControlToolKit is refering 3.5.0.0. And this cause problem.
I just download source code for AjaxControlToolKit , change it reference to refer to same System.Web.Extensions.dll I am using in my project and compile it, and use AJAXExtensionsToolbox.dll I produced, works fine.
You don't have to do this way, you can simple make your project refer to 3.5.00 version of System.Web.Extensions.dll come with .net 3.5 as well.
Or if you are using .net 2.0, you can just go to http://www.asp.net/ajax/ajaxcontroltoolkit/samples/ to download Toolkit version 1.0.20229 (if it is still available there) or download source code you compile yourself.
Anyway, version mismatch is the problem.
Hope it is same problem for you.
Good luck!
- Ryan