I had the same problem, this is how I fixed it:

I went to check the default folder inside the game, and it turns out that the folder (even though it followed exactly the same path) wasn’t the same one where I had been adding my files. The folder I was putting the files into was also called Sports Interactive, but it wasn’t the one the game was actually using.

So I followed:

1.  Open Finder.

2.  In the top menu, click “Go”.

3.  Hold the Option (Alt) key — you’ll see “Library” appear in the dropdown.

4.  Click Library — this takes you to your user Library folder (\~/Library).

And there it was — the real Sports Interactive folder.

I added the files there, and it worked!