Hi Michele,
It seems that you are using the .NET 3.5 version of the RadToolTipManager for ASP.NET AJAX control, but your site does not reference the .NET 3.5 version of the ASP.NET AJAX extensions. Try opening the Property Pages for your web site in Visual Studio 2008 and change the Target Framework property from the Build panel to .NET Framework 3.5.
If you are using Visual Studio 2005 or you need to use the .NET Framework 2.0 for the site, please replace the Telerik assemblies in your project with the .NET 2.0 versions (located in the bin folder of the installation, not in the bin35 folder).
Sincerely yours,
Svetlina
the Telerik team