After you open the game make a save, close it then search for the renpy folder in your library.
The path is, right click on your finder icon in the dock ( choose go to folder...) once a pop up window will appear copy and paste this: ~/Library/
Then on the library folder use the arrow keys to enter the column and search or type renpy and, once you find the folder search for the folder namer named: ProjectMist-1572520246( this how it appear in my Renpy save folder in my Mac;
Be sure that the save inside this folder have the same name of the save you want to use to overwrite it.
Example: you made a save in the slot 1 in the first page of game saves, the save should be named 1-1.save ( this mean you made a save in page # 1 in the slot #1), you can:
Overwrite it with the external save if it have the same name ( just drag and drop it in the Mist folder and the click yes when asked to replace);
If the save have a different denomination you can:
Make a save in the game with the position determined by the denomination of the save and then overwrite it;
Example: the save is named 2-3.save ( in this case the save was made in the page#2 in the slot#3)
Rename the save you want to use
Example: you made a save in the page #1 slot #1 so the save in the folder is called 1-1.save but the save have a different name;
Rename the external save ( right click on it, choose rename) then type 1-1.save ( or the number you desire always based on where you made the save you want to load) then drag and drop the save in the Mist folder and click replace when asked)
Reopen the game and load the save, work done.