So... I haven't solved the problem but i tried to look where the error come from.
So far, the error seems to come from the ligne 1297 in md_megamenu.admin.inc :
$data = drupal_get_form('edit_menu', $mid);

I stop here because i don't know where to find the edit_menu form.

Still need help please,
Gregory

Once again, i tried some things ...

It appears that if i change this line :
$data = drupal_get_form('edit_menu', $mid);
to this
$data = drupal_get_form('md_megamenu_edit_links_form', $mid);
the page doesn't bring a fatal error anymore and i see the admin interface.

That is to say that this function doesn't do its job :
/**
* implements hook_forms()
*/
function md_megamenu_forms($form_id, $args)
{
$forms['edit_menu'] = array(
'callback' => 'md_megamenu_edit_links_form',
);

$forms['configure_menu'] = array(
'callback' => 'md_megamenu_configure_form',
);

return $forms;
}

BUT i can't do anything because the regions where i'm supposed to drop things are not accessible.

Do somebody is here to help or will i debug this myself ?
In that case, why is there a support in the first place ?

I'm really close to cancel my buy ...

In fact, i can't do anything because all the pop windows appears behind the half-transparent grey background.

The last issue was resolved by this post : http://megadrupal.com/forum/module-support/mega-menu/142
By the way, can the mega menu split a drupal menu in two column (or more) automatically ?