Copy freest-mode folder to any folder you like (~/.emacs.d/ in this case) Then add the following lines to your emacs init file:
(add-to-list 'load-path "~/.emacs.d/freest-mode/")
(require 'freest-mode)
;; This automatically chooses freest-mode when editing FreeST files.
(add-to-list 'auto-mode-alist '("\\.fst\\'" . freest-mode))