Here is step by step activities to integrate NUnit with Visual Studio 2012.
1. Install NUnit framework from http://nunit.org/
2. In Visual Studio IDE, Click Tools-> Extensions and Update-> Click on Online Gallery, Search for nUnit and Install NUnit Test Adapter.
3. Add test project to your solution.
4. Add reference to NUnit Framework in test project.
5. Write unit test for the solution.
Now all tests (NUnit and Visual Studio Test Framework) will appear in the test explorer window.