AJAX :: AutoCompleteExtender Won't Drop On Page?

This is my VERY FIRST experience with AJAX, so be as non-elitist and as clear as possible -- mayve even to the point of talking to me like I'm a child.

GIS programmer add an autocomplete search box to her mapping application. Before I can do that, I need to understand how to do it in a simple place first. I'm working with Visual Studio 2008 on my desktop machine and my website files are on a Windows 2003 Server running IIS 6 to which I have a mapped share called X:. Yesterday, I downloaded the AJAX Control Toolkit and added it to Visual Studio. It shows up in my toolbox and I can add and remove items. Cool. I can start an "ASP.NET Web Application" from Visual C# -> Web and I can drop many controls on the page with design view, but the extenders do not drop there. Particularly, I am interested in the AutoCompleteExtender. If I double-click, nothing happens. If I drag it over the design view, I see a slashed circle. If I add it in the source view, it gets highlighted and I'm told "Element 'AutoCompleteExtender' is not a known element. This can occur if there is a compilation error in the Web site, or the web.config file is missing." The web.config is actually not missing.

I have been told by a friend that I may need to add something to my Web.config to enable AJAX stuff, but he offered nothing else on that. I see a system.webServer section in my Web.config and a comment above it that tells me it is not required for IIS versions earlier than 7.

Also, if I drop a TextBox on my page and try to choose an extender, the AutoCompleteExtender is listed in the Extender Wizard; however, adding it this way ends up with the same result.