AJAX ModalPopupExtender Error Unknown server tag 'asp:ModelPopupExtender

[kunal160]

I am having issues trying to implemenet the AJAx ModalPopupExtender into my web app.My application was a VS2005 application. I converted it to VS2010 as a first step. The target framework is 4.0.I downloaded and installed the AJAXControlToolkit.Binary.NET4.On the page that I want to put the ModalPopupExtender, I first placed a ScriptManager.I then placed a panel and added some buttons and listboxes.I then clicked "Add Extender" on the panel's toolbar and selected ModalPopupExtender.Everything "fine" to this point.However when I go and start to change the properties of popupcontrolid to "panel1" and targetcontrolid to "button1"(on my form)...I get a grey box with red and black text stating:Error Creating Control - Panel1_ModalPopupExtender Undknown server tag 'asp:ModelPopupExtender'.WHen i look at the top of the page I do see my REgister:I have tried everything from altering my web.config (per various web articles) to various other changes. No luck.NOTE that it appears to "work" and run....that is I get my popup.Also I can add a non-ModalPopUpExtender...like the calendar to a textbox...and it too will generate an error complaining about the tag, but run.Thoughts?I have a separate issue dealing with ModalPopupExtender and postbacks that I will post in different question.