Configuration :: Registering A Custom Section In Web.config?

I'm getting an ConfigurationErrorsException when trying to load a custom section from my web.config.

I'm trying to register a custom section in web.config to load it later using a "Section Class".
Suppose my section is orderService, and I register it in web.config as shown:
<configuration>
<configSections>
<sectionGroup name="system.web.extensions" type="System.Web.Configuration.SystemWebExtensionsSectionGroup, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35">
</sectionGroup>
<section name="orderService" type="orderService"/>
</configSections>
<orderService available="true" pollTimeout="00:01:00" location="abc">
</orderService>
<appSettings></appSettings>
<connectionStrings/>

Then I try to load it with:

Configuration config = WebConfigurationManager.OpenWebConfiguration(Request.ApplicationPath);
OrderService custSection = OrderService)config.GetSection("orderService"); <== The exception is thrown here (OrderService is a class that inherits from the class ConfigurationSection, and that defines a property for each orderService property, in this case: available, pollTimeout and location)

The exception says: An error occurred creating the configuration section handler for orderService: Could not load type 'orderService'. (my web.config path line 23) Does anybody know what am I doing wrong here? (I got it from the book APress: Pro ASP.NET 3.5 in C#)