My mode for doing this is :

I use the EnterpriseLibrary first off.

using System;
using System.Collections.Generic;
using System.Text;

using Microsoft.Practices.EnterpriseLibrary.Data;

namespace MyCompany.Applications.MyCustomApplication.Data
{
   public abstract class DataLayerBase
   {

       private string _instanceName = string.Empty;

       public DataLayerBase()
       {

       }

       public DataLayerBase(string instanceName)
       {
           this._instanceName = instanceName;

       }

       protected Microsoft.Practices.EnterpriseLibrary.Data.Database
GetDatabase()
       {
           Database returnDb;
           if (this._instanceName.Length > 0)
           {
               returnDb =
DatabaseFactory.CreateDatabase(this._instanceName);
           }
           else
           {
               returnDb = DatabaseFactory.CreateDatabase();
           }
           return returnDb;
       }

   }
}

Then all my DataLayer objects (EmployeeData for example) inherit from that
class.

The .config file is just xml, so you can read it using Xpath statements.

which looks like

\\configuration\connectionStrings\add

XmlNodeList nl = root.SelectNodes ( \\configuration\connectionStrings\add) ;

something ~like that ( I have not tested this, just giving you an idea).