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

You need to add a reference to the NUNIT dll

It seems like you have references missing. From official Selenium docs page:

Add references to the following DLLs: nmock.dll, nunit.core.dll, nunit. framework.dll, ThoughtWorks.Selenium.Core.dll, ThoughtWorks.Selenium.IntegrationTests.dll and ThoughtWorks.Selenium.UnitTests.dll

For your current problem, you need to reference at least NUnit.Framework.dll and Selenium dlls.

Please read the docs page for more info.

Right click on references in the solution explorer and click add reference. You will need to browse for the nunit.framework.dll