The post •https://tfsbuildextensions.codeplex.com/wikipage?title=How%20to%20integrate%20the%20nUnit%20build%20activity gives all the steps required to make use of this community TFS extensions. It is meant to be a step by step to get the activity working
Step 1 of the notes shows how to find all the assemblies that have been built, basically scan a for any file ending *.dll. I think this is the key to your question, you just pass a list of all DLLs produced by the build and let nUnit find the tests. If you want to do something else you can build the list of DLLs any way you want, but a scan seems the most simple, you don't have to worry over managing the list ypourself
Step 2a runs the nUnit test, the list of DLLs is passed into the nUnit activity (in effect the command line test runner). This will look in each DLL for the nUnit attributes and run the tests it finds.
Step 2b publishes the results back to TFS. This is done by taking the XML output of the test runer and publishing it with MSTEST to the TFS server.
If you are using TFS 2012 or later I would not use this activity, I would use the new test adaptor model built into TFS http://blogs.blackmarble.co.uk/blogs/rfennell/post/2012/03/27/Unit-testing-in-VS11Beta-and-getting-your-tests-to-run-on-the-new-TFSPreview-build-service.aspx and http://visualstudiogallery.msdn.microsoft.com/c8164c71-0836-4471-80ce-633383031099/