The type or namespace name 'NUnit' could not be found

I have a c# code.(which is exported from selenium IDE)

using System;
using System.Text;
using System.Text.RegularExpressions;
using System.Threading;
using NUnit.Framework;
using Selenium;

namespace SeleniumTests
{
[TestFixture]
public class csharp
{
private ISelenium selenium;
private StringBuilder verificationErrors;

[SetUp]
public void SetupTest()
{
selenium = new DefaultSelenium("localhost", 4444, "*chrome", "http://localhost:1924/");
selenium.Start();
verificationErrors = new StringBuilder();
}

[TearDown]
public void TeardownTest()
{
try
{
selenium.Stop(); 
}
catch (Exception)
{
// Ignore errors if unable to close the browser
}
Assert.AreEqual("", verificationErrors.ToString());
}

[Test]
public void TheCsharpTest()
{
        selenium.Open("/cookie/Default.aspx");
        selenium.Type("id=TextBox1", "ranadheer");
        selenium.Type("id=TextBox2", "SE");
        selenium.Type("id=TextBox3", "hyderabad");
        selenium.Click("id=Button1");
        selenium.WaitForPageToLoad("30000");
        selenium.Click("id=Button2");
        selenium.WaitForPageToLoad("30000"); 
}
}
}

I pasted this code in a console application (visual studio 2008).
After running, I got this errors:

The type or namespace name 'NUnit' could not be found.
The type or namespace name 'TestFixture' could not be found
The type or namespace name 'ISelenium' could not be found
The type or namespace name 'SetUpAttribute' could not be found

What framework should I add to correct this errors?