I don't have a Mac device, so I can't tell you for sure. Doesn't Mac OS have an Applications folder where all your program files are in? It indeed sounds like you just found the secondary save location.
I didn't know that, thanks for the info! ...Though I didn't want to give up so I figured out that "app" files are archives that you can open with programs like 7zip. In the directory "Contents/Resources/autorun/game" there's the same folder structure as in the Windows version and you can copy the mod file into it, though it takes a while to process. Would you mind testing whether the game actually runs afterwards on your Mac device?
I can't guarantee that this won't break the executable though, so either make a backup of it or prepare to redownload it. I'd be very thankful if you could test this for me but you don't have to :P
No, the Contents folder is in the "app" file. This file can also act as a folder, if you have the right program to open it! On Windows I used 7zip to do it, but that program is exclusive to Windows, so you'd need to look for an "archiver" program that runs on your operating system. I don't have any recommendations for that I'm afraid.