Go to visual studio package manager console.
To install specflow for NUnit, type
Install-Package SpecFlow.NUnit
This will install all the required packages and a correctly configured app.config to work with NUnit.
If you instead resolved all the references by hand, then you would still have to construct the app config correctly. Finding the right config for MSTest or NUnit can take a little while to get right so it is better to just use the nuget package.