Oh, gosh... I realize that I'm not sure how the "saves" folder works on the Mac now that you're mentioning this. I had assumed it was the same as on PC, but apparently it's in a different spot if you can't find it??
OH but it looks like RenPy still does the appdata saves then on Mac! RenPy actually saves your games in two different places—the saves folder and your appdata on your computer. It can sometimes be not foolproof, which is why I always recommend copying over the saves folder to make sure everything is there, but it looks like yours is working fine! If you were able to see all your save files and start Arc3 even without moving anything, then you should be good to go!!! (And I will still go and look into the save folder thing on Mac versions...)