Emacs mode can be chosen automatically on the file's name. To edit file ended by .stk or .stklos in Scheme mode, you have to set the Elisp variable auto-mode-alist to control the correspondence between those suffixes and the scheme mode. The simpler way to set this variable consists to add the following lines in your .emacs startup file.
;; Add the '.stk' and '.stklos' suffix in the auto-mode-alist Emacs ;; variable. Setting this variable permits to automagically place the ;; buffer in scheme-mode. (setq-default auto-mode-alist (append auto-mode-alist ("\\.stk$" . scheme-mode) ("\\.stklos$" . scheme-mode)))