Excellent, it works !
So finally, my code (that i added in the main.php file) looks like that :
<?php global $ID; ?>
<form class="button btn_edit" method="post" action="/bdrt/dokuwiki/doku.php?id=<?php echo $ID?>">
<div class="no">
<input type="hidden" name="do" value="edit" />
<input type="hidden" name="rev" value="" />
<input type="image" value="Modifier cette page" class="button" border="0" src="/bdrt/dokuwiki/lib/exe/fetch.php?w=20&media=wiki:modifier_24.png" />
</div>
</form>
I've done the same for the buttons "revision", "media manager", "Administration" and "Log out".
Thanks !
Carole