This took forever. I had to pull a lot of resources but I was finally able to do it in just a few simple steps.
First, go ahead and boot up Minecraft and everything.
(Not sure if you have to but just in case)
Download whatever map you want and open that zip and have that folder ready.
Now press shift+command+G on your keyboard.
(Make sure you have everything closed out so your on the finder application, but dont open finder! You cant find this on finder)
Now, a little box should appear. It might be blank or it might now. Make sure its blank.
Type this exactly into that box:
Perfect! Now you've got the minecraft folder open.
Open the "saves" file.
Now drag your map files that you opened up from the zip and place them into the saves folder.
Now open minecraft! Your maps should be there in single player.