There are few steps that helps you in configuring ajax control toolkit in Visual Studio 2008::
1)Download AjaxControlToolkit-Framework3.5.zip from http://www.codeplex.com/Release/ProjectReleases.aspx?ProjectName=AjaxControlToolkit&ReleaseId=8513
2)Create ASP.NET Web Application project.
3)Go into Toolbox, right click and choose Add Tab. Name it as Ajax Control Toolkit. Click on choose items and click on browse button. Then give the location of download file and find a folder SampleWebSite and go inside bin folder, then chose AjaxControlToolkit.dll and click ok.
4) After the third step, you can see the appearance of Ajax Control Toolkit and its components in Toolbox.
5) Now Enjoy the Ajax Control Toolkit.
