I am a macbook owner and i have problems with downloading maps to the game. Just to specify my problems, i cant find the folder for minecraft. Well i have found it, but theres nothing else in there but the game itself.
2. The easiest way (IMO) to find the saves folder: Open Minecraft, click on "Texture Packs" and then "Open Texture Pack Folder". That should open a folder called texturepacks. Go up one level from that folder and you should be in a folder called .minecraft. In there will be a folder called saves which is where you need to copy any custom maps to. Make sure they're not in a zip or rar file.
Let me know if this works -- I play on PC and Linux so I'm not 100% sure it will work on Mac.