As Scott said, the above instructions were for Moodle 1.9.
On Moodle 1.8, I'm not sure you can override the files from within the Moodle UI.
So you could edit the lang files directly or you could also try creating a new en_utf8_local (under /mod/facetoface/lang/)directory and putting a copy of the facetoface.php file in there with only the strings you want to override.
Francois
Face-to-face (plugin)
Editing Confirmation notifications
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