Error Creating ASP Server Tag

Question

In haveing my asp.net .sln.  I am doing the error "Error Creating ASP Server Tag". 

I will be editing web.config to including

<add tagPrefix="ajaxToolkit" assembly="AjaxControlToolkit" namespace="AjaxControlToolkit" />

how its not working still.

Does anyone have any happinesses for error?

Thans,

Shusta