How do I have to access to web.config appSettings section from my CallOut class inherited from CrmCalloutBase?

In my code below, I only get MyConnectionString setting defined in CRM's root directory CRMWeb

public class AccountCallout : CrmCalloutBase

{

    public AccountCallout()

    {

        s = (string)System.Configuration.ConfigurationSettings.AppSettings["MyConnectionString"];

    }

}

What I have to do to access to CRM web.config appsettings section?

What am I doing wrong?

In a regular WebApplication there's no problem to access web.config appsetings section from a dll.

I'm using Microsoft CRM 3.0 and Visual Studio 2003.

Thanks.