Crystal Reports Report Source problem

Revision 2

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using CrystalDecisions.CrystalReports.Engine;
using CrystalDecisions.Shared; 


namespace Reports
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void btnSimpleReport_Click(object sender, EventArgs e)
        {
            NorthwindDataSet dts = new NorthwindDataSet();
            NorthwindDataSetTableAdapters.CustomersTableAdapter dta = new NorthwindDataSetTableAdapters.CustomersTableAdapter();
            dta.Fill(dts.Customers);
            RPTsimple report = new RPTsimple(); //create instance
            report.SetDataSource(dts);
            crystalReportViewer1.ReportSource = report; //Problem Line
        }

Why is ReportSource underlines in visual studio. Im using C# framework 3.5
look at //problem line
my error:

'Microsoft.Reporting.WinForms.ReportViewer' does not contain a definition for 'ReportSource' and no extension method 'ReportSource' accepting a first argument of type 'Microsoft.Reporting.WinForms.ReportViewer' could be found (are you missing a using directive or an assembly reference?)