Drush not working on Mac OSX
A "possible" reason might be because Drush commands must be issued from the appropriate subdirectory of your website (change to the directory where your Drupal site is installed and try again).

So try navigating to the sites/default folder (assuming this is NOT a multi-site), and then retry your Drush comand.

If that doesn't help, it could well be an issue with the version of Drush that you are using. Would it be an option to downgrade to a lower version of Drush? FYI: Drush 6.x does support D7 also ...