This is because Mac apps are basically folders in disguise. It's looking for a manifest to tell it which executable to run, but the DLC doesn't have either. You need to manually install the contents into the original app. I'll run you through it.
1. Download the DLC.
2. Locate the original app and the downloaded file.
3. Right-click on each and select 'Show Package Contents'. In both cases, you should now see a folder called 'Contents'.
5. For both, navigate 'Contents -> Resources -> Autorun -> Game'. You should see a bunch of files in one folder, and in the other a single file called 'sharks.rpa'.
6. Drag 'sharks.rpa' from the DLC's Game folder into the original app's Game folder.
That's it! Launch the game and go spearfishing, you should find you have the DLC.