The easiest way to get the folder you need to save on macOS:
open terminal (you can find it in Applications/Utilities or else just use the spotlight search)
copy the following line and paste it into terminal
mkdir ~/Library/Application\ Support/Nano-control
hit enter
This creates a folder named Nano-control in ~/Library/Application Support, which will allow the game to store save files there.
The actual easiest would be to run it in a .command script (so that all you have to do is double click it from the finder) but macOS throws a security fit when you try to run a downloaded script.
First off all downloaded scripts are tagged as "don't run me" so when you try to run them it throws a warning and then you have to go into the security preferences and tell it to run anyways.
Secondly when I upload the script and redownload it macOS automatically sets the permissions for the script to read-only (as opposed to execute).
The instructions to fix this are longer than the above instructions and since you only have to run the script once y'all lewd mac gamers are just gonna have to do a little terminal legwork. You should be used to that by now, anyways.
@speedapple Ideally you'll want to include this instruction as part of the startup sequence for the game. RPGM should have some way for you to make a directory (Nano-control) at a certain file-path (~/Library/Application\ Support/), although I have no idea what it is. Just make sure it isn't going to overwrite anything when it does, because eliminating people's save games will probably make them cranky.