Having problems with AJAX not working with your DNN Modules? Receiving an "Object reference not set to an instance of an object" error when you try to register the toolkit? There have been a few issues with trying to run the latest version of the AJAX toolkit with DotNetNuke 5, try running build 3.0.30930.28736 instead of the latest build. It doesn't require you to register the toolkit and, it is available for download here http://ajaxcontroltoolkit.codeplex.com/releases/view/33804
Just place the DLL in your DNN bin folder, add register the assembly at the top of which ever control (in the module) needs AJAX and you are ready to go.
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="AjaxToolkit" %>