AjaxControlToolkit and web.config

Hello I'm beggining with AjaxControlToolkit but my problema is that I can't find nothing about the web.config,

I have downloaded an example that is working. but has and old ajaxcontroltoolkit.dll, but If i change with an ajaxcontroltoolkit  for asp.net 4, I get an error.

In the example web.config I have several lines refering the ajaxcontroltoolkit versión 3.5.

I have read in the https://ajaxcontroltoolkit.codeplex.com/ but I can't find nothing about web.config.

Thanks