Once the NUnit Framework DLL is referenced you are ready to write test cases.
Working with NUnit
There are a number of unit testing frameworks for .NET, but by far the most widely used is NUnit. After installing NUnit, the only thing you need to do to start unit testing is to include the nunit.framework.dll in your project's references. To do this, simply right click on the References node of the solution explorer in Visual Studio .NET and browse for the DLL file. By default, this is installed under C:\program files\nunit 2.2\bin\.

Adding a reference to the NUnit Framework DLL file provides access to unit testing for the current Visual Studio .NET project.
Once the NUnit framework is available in a Visual Studio (VS) project, you are ready to begin coding your tests. Before we get into that, however, it's worth noting that NUnit comes with two test runner applications. One is a console version and the other a Windows GUI. While the GUI is most commonly used when running tests manually, the console version provides a way to automate testing with batch files or other scripts that can capture the results in unattended mode. Both test runners, however, require you to leave the Visual Studio IDE in order to run your tests

The NUnit GUI shows a graphical representation of the unit test results for your application.

While this is perfectly acceptable, there is a third-party freeware add-in for Visual Studio called TestRunner. This handy little utility is fully NUnit compatible and lets your unit tests be executed from within the programming environment.