solved, i didi it
in index.php :
<div id="s5_menu_wrap">
<?php if ($s5_show_menu == "show") { ?>
<?php include("vertex/s5flex_menu/default.php"); ?>
<?php } ?>
<?php if($s5_pos_search == "published") { ?>
<div id="s5_search">
<?php s5_module_call('search','notitle'); ?>
</div>
<?php } ?>
<div style="clear:both; height:0px"></div>
</div>
changed into :
<div id="s5_menu_wrap"><div id="menucontainer">
<?php if ($s5_show_menu == "show") { ?>
<?php include("vertex/s5flex_menu/default.php"); ?>
<?php } ?></div>
<?php if($s5_pos_search == "published") { ?>
<div id="s5_search">
<?php s5_module_call('search','notitle'); ?>
</div>
<?php } ?>
<div style="clear:both; height:0px"></div>
</div>
and modifed the css lines to center the good div