Problem with running NUnit tests

I have created a build configuration, which runs tests in NUnit framework.

In these tests we use ConfigurationManager class to get values from App.config file.

When I run these tests manually, everything works.

However, when I run these tests using TeamCity, either no value can be found in app.config file or the app.config file is not recognized. I any case, I get an error" "the value cannot be null". 

We get the values from app.config in this way:

ConfigurationManager.AppSettings["NameOfProperty"]

 

Can someone please tell what the problem is?