I am not able to run the unit tests in Visual Studio 2013 like I used to be able to run it from the Visual Studio window itself in Visual Studio 2010. I
Unfortunately this is not possible. The Express SKUs of Visual Studio do not support plugins / extensions.
Try to use NUnit TestAdapter including NUnit. You will find it searching for NUnit in the Manage NuGet Packages window.
I have tested it, and it works fine.
A workaround is in blog post How to invoke NUnit from Visual studio express and execute automation tests.