Not getting ConfigurationManager class in my dal.

Not getting configurationmanager class. Actually im using Sqldatasource. please once chek my dal
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using pnGridml;
using System.Data.SqlClient;
using System.Configuration;
using System.Data;


namespace pnGridDal
{
public class pnGrdDal
{

public static List<Product>GetProducts()
{
return GetProducts(int.MaxValue,0);
}
public static List<Product>GetProducts(int maximumRows,int startRowIndex)
{
//return GetProducts(maximumRows,startRowIndex);

string sql=@"SELECT [ProductName],[UnitPrice],[UnitsInStock],[QuantityPerUnit] FROM [Product]";
Here i want like this. Bt not getting.......................................... (using (SqlConnection myConnection = new
SqlConnection(ConfigurationManager.ConnectionStrings[
"NWConnectionString"].ConnectionString)))


}

}
}