Looks like your symlink is a finder alias and not a symlink. OSX Finder handles both of them the same way, but at system level the finder alias is a plain file.
Substitute the finder alias for a proper unix symlink using ln -s command and it should work.
As mentioned in the IDO documentation, you can place the following inside your Emacs init file.
(ido-completing-read "M-x " (all-completions "" obarray 'commandp))))
Make sure you have already setup ido-mode correctly, before settings the above global ...