<cc1:modalpopupextender runat="server"></cc1:modalpopupextender>

what is missing or any guesses... 

Has this been encountered and how is this resolved?

 if you add the following in your web.config ,then you can use the tagprefix <ajaxtoolkit:modalpopupextender>

<controls>
                 <add tagPrefix="ajaxtoolkit" namespace="AjaxControlToolkit" assembly="AjaxControlToolkit"/>
      </controls>

thanks for the starting point.

however the assembly has no entry and is unresolved therefore the knock-on is the cc1 goes to now be named ajaxtoolkit but the issue still remains that the control is not recognized.

how does this get resolved?

 Please make sure that you add the dll to the bin directory of the web application

Hi,

I agree with the member above.

Firstly, please make sure you have added a reference to AjaxControlToolkit.dll in your project and then you can try to register it in you page with directive to see if it works.

For example:

<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>

I hope this helps.