Your comment says you've resolved this. Something tells me you did it by adding
(add-to-list 'auto-mode-alist '("\\.m" . matlab-mode))
to your .emacs
. I've got so many of these sprinkled around that I just wrote a convenience macro for it:
(defmacro by-extension (ext mode)
`(add-to-list 'auto-mode-alist '(,(format "\\.%s" ext) . ,mode)))
which lets me write things like
(by-extension "m" matlab-mode)