All you are doing is editing 2 text files which can be done on a Mac. From Adapt's link but Mac-ified:
In the Finder,
Open Go To Folder from the Go menu
Copy & Paste the following path into the box:
~/.config/StardewValley/Saves
and click Go
It'll open up Stardew Valley's Saves folder where you can find a folder for each of your Farmers with his/her name and a serial number attached.
**I would immediately Copy your Farmer folder of interest and Paste a copy somewhere else for safekeeping, like into your Documents folder or similar.
In the folder of interest, there should be 2 files with the name of your character with a string of numbers after them, open both in TextEdit (I've also been editing the .old file just to be safe.)
Hit Cmd + F and enter <whichFarm> it should bring up something similar to <whichFarm>0</whichFarm> All you have to do is edit the number in the middle.
0 = Standard Farm
1 = River farm
2 = Forest Farm
3 = Hill-top farm
4 = Wilderness farm
Edit BOTH files with the same number and save them.
Then just open your game, then that farmer and you're good to go.
I tested this on my PC and the search ID's my farms correctly but I haven't attempted the edit to switch farm types yet. It should work the same on your Mac.
EDIT: OK I just did this and it worked. However, you will need to clean up your farm first because (in my case) the Hill Top farm has farmable locations which in the Standard farm is in the ponds. So some of my crops are inaccessible and it looks messy of course. I need to restore my original save folder I put in my Documents folder, remove those first, and repeat the process to make sure the Standard farm looks normal. Same would happen for buildings, so a bit of cleanup needs to happen first.