VS2010添加了System.Configuration引用为什么找不到ConfigurationManager？

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data;
using System.Data.SqlClient;
using System.Configuration;
using NewsModels;
namespace NewsDAL
{

public static class UserService
{
    private  static string connectionString = ConfigurationManager.
}

}

看看项目的net framework版本