In a recent project, I decided that I would use NUnit for testing functions during my development. There were a couple of things I learned in using it within Visual Studio 2010.
First, I read that the Nunit.config tags were needed within 2010 - as it defaults to .Net 4. NUnit installs to the "C:\Program Files\Nunit<version>". Within that is the "bin\.Net-2.0 folder. There is a config file "nunit.exe.config" that is used when using the GUI for testing. right after the configuration tag I read that you need to add:
<startup>
<requiredRunTime version ="4.0.30319"/>
</startup>
I am using Nunit 2.5.9 - and
without those tags - it still worked for me.
The other thing I learned was what would normally be an App.Config file needs to be a totally different file. You need to create a config file that matches the NUnit Project file name - and it must be located in the same directory as the NUnit Project file - so the NUnit GUI can find it.
Our project structure has a folder for Dependencies where we store DLLs that get used in the project - but might not have the source in the project. I created my NUnit Project file in that folder. If that file is called NUnitProjectTests.nunit - there must be a config file called NUnitProjectTests.config - and that is used as your App.Config while running the GUI tests. In my project it is not in the "bin\Debug" folder - it's in a totally different folder.
You just need to remember to set the values in there and make them match your real app.config for your project output.