backup,sqlserver,SQLDMO都找不到类型或命名空间

以下是我的代码
using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;
using System.Data.SqlClient;
namespace WindowsApplication6
{
/// <summary>
/// Form1 的摘要说明。
/// </summary>
public class Form1 : System.Windows.Forms.Form
{
private System.Windows.Forms.Button button1;
/// <summary>
/// 必需的设计器变量。
/// </summary>
private System.ComponentModel.Container components = null;

public Form1()
{
//
// Windows 窗体设计器支持所必需的
//
InitializeComponent();

//
// TODO: 在 InitializeComponent 调用后添加任何构造函数代码
//
}

/// <summary>
/// 清理所有正在使用的资源。
/// </summary>
protected override void Dispose( bool disposing )
{
if( disposing )
{
if (components != null)
{
components.Dispose();
}
}
base.Dispose( disposing );
}

#region Windows 窗体设计器生成的代码
/// <summary>
/// 设计器支持所需的方法 - 不要使用代码编辑器修改
/// 此方法的内容。
/// </summary>
private void InitializeComponent()
{
this.button1 = new System.Windows.Forms.Button();
this.SuspendLayout();
//
// button1
//
this.button1.Location = new System.Drawing.Point(120, 72);
this.button1.Name = "button1";
this.button1.Size = new System.Drawing.Size(96, 32);
this.button1.TabIndex = 0;
this.button1.Text = "button1";
this.button1.Click += new System.EventHandler(this.button1_Click);
//
// Form1
//
this.AutoScaleBaseSize = new System.Drawing.Size(6, 14);
this.ClientSize = new System.Drawing.Size(292, 266);
this.Controls.Add(this.button1);
this.Name = "Form1";
this.Text = "Form1";
this.ResumeLayout(false);

}
#endregion

/// <summary>
/// 应用程序的主入口点。
/// </summary>
[STAThread]
static void Main()
{
Application.Run(new Form1());
}

private void button1_Click(object sender, System.EventArgs e)
{
//引用SQLDMO.dll，SQLDMO由Microsoft SQL Server自带的SQLDMO.dll提供，SQLDMO.dll是一个COM对象
SQLDMO.Backup backup=new SQLDMO.BackupClass();
SQLDMO.SQLServer sqlserver=new SQLDMO.SQLServerClass();
sqlserver.LoginSecure=false;
sqlserver.Connect("localhost","sa","1");
backup.Action=SQLDMO.SQLDMO_BACKUP_TYPE.SQLDMOBackup_Database;
backup.Database="pubs";
backup.Files=@"E:\Project\meng\data\userdb.bak";
backup.BackupSetName="userdb";
backup.BackupSetDescription="数据库备份";
backup.Initialize=true;
backup.SQLBackup(sqlserver);
}
}
}