Hi:
There is a trick to using NUnit. In an article by Paul Kimmel called _Debugging .NET with NUnit_ from Mar 14, 2003 he states:

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Recall that NUnit creates an instance of the AppDomain for the .DLL you are testing. One of the other things it does is to manually look for an assembly.dll.config file. Thus, if you need configuration options for your application, copy those settings into a .config file for testing. In our example, such a .config file would be MyMath.dll.config, placed in the same directory as the test DLL loaded by NUnit.
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Thus in my situation where I was attempting to test mmApplication I needed
to copy app.config to the /bin/debug folder and rename it mmApplication.dll
(config extension is hidden). This solved the problem. Sorry to use so
much bandwidth - hope this can be helpful to others.

the full article can be found at:
www.informit.com