Run M-x drupal-spell-enable to setup the Drupal dictionary as an extra dictionary in that buffer.
The command will be enabled automatically when activating
drupal-mode.
The easiest way to install Drupal spell is probably to install it via the ELPA packages at Marmalade or MELPA.
ELPA (package.el) is part of Emacs 24. For Emacs 23 see Marmalade for installation instructions.
You need to use aspell as your spell checker as
ispell does not support the use of extra dictionaries.
Drupal spell is actively developed at GitHub. Feature requests, ideas, bug reports, and pull request are more than welcome!