The directory structure in the Mac OS is seen by Emacs as
So when Emacs requests a file name, doing file name completion on / will display all volumes on the system. As in Unix, .. can be used to go up a directory level.
To access files and folders on the desktop, look in the folder Desktop Folder in your boot volume (this folder is usually invisible in the Mac Finder).
Emacs creates the Mac folder :Preferences:Emacs: in the System Folder and uses it as the temporary directory. The Unix emulation code maps the Unix directory /tmp to it. Therefore it is best to avoid naming a volume tmp. If everything works correctly, the program should leave no files in it when it exits. You should be able to set the environment variable TMPDIR to use another directory but this folder will still be created.