Please can any one help in the following error.

I added the AirlineReservationsLib class Library but it didn't allow me use the following line:

string connString = System.Web.Configuration.WebConfigurationManager.C onnectionStrings["airlines"].ConnectionString;

It is giving me the following error:
The type or namespace name 'Configuration' does not exist in the namespace 'System.Web'

I'm getting another error when I try to add the reference "using System.Web.Configuration"

Can any one advise what I'm missing?

Thanks in advance