To make sure files with extension .rec use rec-mode, add the following line to your .emacs file.
(add-to-list 'auto-mode-alist '("\\.rec\\'" . rec-mode))
rec-mode buffers need font-lock to be turned on - this is the default in Emacs1.
With this setup, all files with extension ‘.rec’ will be put into rec mode. As an alternative, make the first line of a recfile look like this:
# -*- mode: rec -*-
which will select rec-mode for this buffer no matter what the file's name is.
[1] If you don't use font-lock globally,
turn it on in the rec buffer with (add-hook 'rec-mode-hook
'turn-on-font-lock)