It depends on which part of the game that save file is from, but it usually is not a good idea to use the Windows save files on Mac and Linux.
If you still want to try them, here is the path to the save files:
~/Library/Application Support/freebirdgames/tothemoon/
The ~ at the start means the Library folder available in your user's folder.
That Library folder is hidden by default on OS X 10.7 and later, in order to access it you can use different methods, one of them is pressing Command-Shift-G which is the "Go to folder" shortcut in Finder, then paste the above path in it and press Go.
Please don't forget to:
1- Backup your existing save files.
2- Rename the save files you download from this topic to save1.rxdata, save2.rxdata, etc.