What they're getting at in that other question is that when you add a file to a project, it's not necessarily added to the current target. You can have multiple targets in a project, and you can add files that don't get compiled (e.g. developer documentation), so it's possible to add a file but then have it just sit there and not get built.
If you select the file in Xcode and open up the Utilities pane on the right, there's a section that shows target membership. It looks like this:
This one shows that the file belongs to the project momdecTests
but not to the project momdec
. Make sure your app target is checked here.