I don't own a Mac, so can't really say for certain, but it looks like you should be able to find the save files this way:
Your Ren'py save files on Mac can be found at this path:
~/Library/RenPy/game_directory
To access the above path:
Open up a Finder window
Press COMMAND + SHIFT + G
Paste/enter the path (~/Library/RenPy/game_directory) into the text field and hit Go
Hopefully that helps!