The NUnit Test Adapter only runs NUnit 2.X tests in Visual Studio 2012 and newer.
Note that it does not run NUnit 3.X tests. For that, you need the NUnit3TestAdapter
Please consider moving your code to use NUnit3. This adapter is only maintained to support newer versions of Visual Studio, and is very rarely updated. We might fix an occasional bug if it is critical.
You can download the latest release version
from 
Ask support questions 
The NUnit Test Adapter is Open Source software released under the 
The NUnitTestAdapter wiki is the best place to start.
Also check the release notes.