[OS X Emacs] Use "Open" command in Dired Mode

[Johannes Brauer]

Hi!

I gave the tip from Aquamacs wiki a try. But evaluating this:

(define-key dired-mode-map "o" 'dired-open-mac)
  (defun dired-open-mac ()
     (interactive)
     (let ((file-name (dired-get-file-for-visit)))
       (if (file-exists-p file-name)
           (shell-command (concat "open '" file-name "'" nil )))))

I get the error message: Symbol's value as variable is void: dired- 
mode-map.
Is the tip outdated?

Johannes