Okay, I'm writing this from my memory since I don't have Mac on hand.
Use the finder and use go to folder option. Type in /Applications (should be the real location of Yorna. If you used my recommendations it should be here) and find the game. Right click on the icon and choose "Show Package Contents". Navigate the new window to find the save folder (should be inside Contents\Resources\app.nw) and copy it somewhere else. Install the new release and, using the same way as before, place the save folder into the new release.
I'm pretty sure there are more convinient ways to do this but I'm not a Mac user so I just descrived what I did during the test runs in the past.