Back in January, I had a problem adding the Ajax Control Toolkit to my Toolbox with Visual Studio 2008. The Ajax Controls where grayed out after executing a show all (right mouse click inside Toolbox, select Show All). My solution: I unplugged my Microsoft Wireless mouse. After unplugging my mouse, the Ajax Controls appeared. For more information, follow this link http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=2694733&SiteID=1, which contains a posting of my solutiion.
I also did some follow-up research, another possible fix is to make sure your solution is pointing to the 3.5 .Net framework.
John
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