Modal Popup Extender Won't Insert onto Page

I am trying to use a modalpopupextender in visual studio 2008 and it won't alllow me to insert it onto a page.  When I try to drag and drop it on the page I get the circle with a line through it.  Alot of other controls from the Ajax Control Toolkit give me the same problem.  Some work and some don't.  Any suggestions?
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I'm assuming you're in design mode?
Not really sure why they don't work in there, but if you switch to the source view, you should be able to drop it into the markup properly.

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