From http://amitp.blogspot.com/2012/10/emacs-helm-for-finding-files.html

find-with-mdfind.sh

#!/bin/bash
# Note: updated 2014-03-28 to handle patterns like foo/bar.txt
suffix=$(basename "$@")
pattern="${suffix}*"

if [ "$@" = "$suffix" ]; then
    # If there's no folder then it should be a substring
    pattern="*${suffix}*"
fi

mdfind -onlyin $HOME "kMDItemFSName = '$pattern'cd" \
    | fgrep -i "$@" \
    | grep -v "$HOME/Library/" \
    | grep -v "$HOME/Pictures/" \
    | grep -v "$HOME/Music/" \
    | sed -e "s|^$HOME|~|"

init.el

(require 'helm-files)
(setq helm-idle-delay 0.1)
(setq helm-input-idle-delay 0.1)
(setq helm-locate-command "locate-with-mdfind %.0s %s")
(loop for ext in '("\\.swf$" "\\.elc$" "\\.pyc$")
      do (add-to-list 'helm-boring-file-regexp-list ext))
(define-key global-map [(alt t)] 'helm-for-files))