Your System Under Test may require a .NET application configuration (app.config) file. Since the FIT tests will be run by a different executable than your System Under Test uses in production, you have to specify the app.config file in a Suite Configuration File.

<suiteConfig>
    <fit.Settings>
        <appConfigFile>myDomain.dll.config</appConfigFile>
        ...
    </fit.Settings>
    ...
</suiteConfig>