Sorry to keep bothering,
I fixed the folder name to match the .tmd filename
The path for the .TMD file also seems to be correct. '/Users/username/Library/Containers/com.aerofly.aerofly-fs-4-mac/Data/Library/Application Support/Aerofly FS 4/addons/aircraft/j3cub/j3cub.tmd'
I also checked the tutorial website. I searched for a file without the /Containers/com.aerofly.aerofly-fs-4-mac/ and none exists.
I don't think it's an issue with the mod itself, as I tried a few of the more reputable ones and they didn't work either.
Please bear with me because I'm not super familiar with modding on Mac.