<p>I've decided to integrate NUnit with VWD2008.</p>

<p>I did the following-
1) Installed NUnit - Ran a Sample project that was included with the installation all the tests were fine.
2) Installed TestDriven.Net 2.0 - Personal distribution.
3) I have written on an MVC Project a test and when I try to right click the context and run the Test it fails.
In addition I had to add reference the NUnit.framework dll in order to write the tests.</p>

<p>What have I done wrong?</p>

Testdriven.Net doesn't work with the express editions of visual studio. Previous versions of Testdriven.Net did work with the Visual Studio Express editions but Microsoft didn't like that and put their lawyers on it.
http://weblogs.asp.net/nunitaddin/archive/2007/07/06/microsoft-amp-testdriven-net.aspx
You could try sharpdevelop if you want a free ide with unit-testing support.