the error means that that file is missing. that means you either did not upload it or you uploaded it to the wrong place.
see if you can follow these instructions exactly;
you need to upload all of the files again.
make sure that you have removed the config.php file from the phpBB3 folder on your computer.
then, on the server side of filezilla, make sure you have just the public_html folder showing in the top window.
on the left side of filezilla ( your computer ) make sure you have the folder named phpBB3 showing. not the files/folders inside it,
just the folder named phpBB3
now, using your mouse, drag and drop the complete phpBB3 folder over to the right hand side so that it will overwrite the phpBB3 folder on the right.
do exactly that, don't do it any different than I posted.
once all of the files have finished uploading do this:
in your browser, go to http://www.temoindemont.org/phpBB3/install/
click on the update tab, then click on update database, then click on submit when it says update database only.
then, go back to ftp and delete the folder named install.
then you are done.
if you do this right, it will all be updated properly and will work properly.