The toolbox in VS 2010 is filtering out the Ajax Toolkit controls that are not applicable for the current project based on the framework the project is using. It will show the latest framework controls that are applicable to the project. For example, if you have 3.5 and 4.0 Toolkit, then in a 3.5 project, it will show only 3.5 toolkit controls, but in a 4.0 project, it will show only 4.0 controls. If you have only 3.5 Toolkit, then both 3.5 and 4.0 projects will show 3.5 toolkit controls. Hope it's clear.

http://weblogs.asp.net/yousefjadallah/archive/2010/04/16/installing-ajax-control-toolkit-4-in-visual-studio-2010.aspx

You could add the Ajax Control Toolkit to the VS Toolbox step by step at:http://www.asp.net/ajaxlibrary/act.ashx.