url mappings causing error page to display

I am testing out url mapping on an ASP.NET 2.0 site that only contains a default.aspx and a default.aspx.cs. 
All it does it displays a couple of messages. It runs okay if I don't have a Web.Config file and it also runs okay if I just have the default Web.Config that is created for the website in Visual Studio 2005.

However, if I add a urlmapping such as:

    <urlMappings enabled="true">
      <add url="~/test.aspx" mappedUrl="~/default.aspx?tab=test" />
    </urlMappings>


I get the following error page. 

Details: To enable the details of this specific error message to be viewable on remote machines,
please create a <customErrors> tag within a "web.config" configuration file located in the root directory of the current web application.
This <customErrors> tag should then have its ...