Hello Half-Note, downloading items and markings on Mac is relatively the same as downloading them on Windows, but locating the FH folder is a little different. I'm assuming that you are using PlayonMac. Open the PlayonMac menu and click, but do not open, FH. Head to "open the directory" and Feralheart's main folder will open. That is where you will place all of the downloads.
(https://i.imgur.com/pzFX9Yl.png)
Check out these links for the download links specifically: https://feral-heart.com/smf/index.php?topic=66500.0 and https://feral-heart.com/smf/index.php?topic=66501.0
For items, all you have to do is download the .zip folder from Mediafire (which might not pop up immediately after clicking the download link---if this happens, wait for the "skip ad" button in the upper-right-hand corner to appear), extract/open the file, and place the items.cfg file directly into the main FH folder, while placing all the items into Feralheart/media/items. Replace existing files when prompted for both.
For markings, do the same as above, but place the markings.cfg file into the main FH folder, and place the individual markings into Feralheart/media/textures/markings. Replace when prompted.
If you need a clearer explanation or get stuck, I will gladly take screenshots of the entire process. Hope everything works out!