Add the following to your .emacs.
(add-to-list 'load-path "~/.emacs.d/plugins/drupal-mode-el/")
(require 'drupal-mode)
(add-to-list 'auto-mode-alist '("\\.\\(module\\|test\\|install\\|theme\\)$" . drupal-mode))
(add-to-list 'auto-mode-alist '("/drupal.*\\.\\(php\\|inc\\)$" . drupal-mode))
(add-to-list 'auto-mode-alist '("\\.info" . conf-mode))