assuming you have put the 'prolog.el' file in ~/emacs_modes, add the following to your .emacs file (in your home directory)

;; adding load path to files
(setq load-path (cons "~/emacs_modes" load-path))

;; PROLOG
(autoload 'run-prolog "prolog" "Start a Prolog sub-process." t)
(autoload 'prolog-mode "prolog" "Major mode for editing Prolog programs." t)
(autoload 'mercury-mode "prolog" "Major mode for editing Mercury programs." t)

(setq auto-mode-alist (cons '("\\.pl" . prolog-mode) auto-mode-alist))