Ok, I think I know the problem. Go to finder and press command + shift + g. You want to type your username, so my username is jd, so I would type:
/Users/jd/Library/
Inside Library create a folder called Frameworks. Next go to https://www.libsdl.org/download-2.0.php and download SDL2-2.0.7.dmg. Take the SDL2.framework file and put it into the Frameworks folder. I think that should work.
Don't put anything in your base. All you should have is KotF 2.1 SP, KotF 2.1 MP, and the KotF folder.
The New Order's post in Help for a Mac user? was marked as the answer