Then this is because the plugin has not been updated to 1.6 properly; it has the old style ACP URLs an a setting on your host is denying access to it 9some servers would do this with the old style URLs, which is why we changed them in 1.6). You need to make the following fixes.
Line 205:
'link' => 'index.php?module=config/menumanager'
to:
'link' => 'index.php?module=config-menumanager'
Line 233:
'link' =>'index.php?module=config/menumanager',
to:
'link' =>'index.php?module=config-menumanager',
Line 239:
'link' => 'index.php?module=config/menumanager&action=add',
to:
'link' => 'index.php?module=config-menumanager&action=add',
Line 252:
$form = new Form("index.php?module=config/menumanager&action=update", "post");
to:
$form = new Form("index.php?module=config-menumanager&action=update", "post");
Line 271:
$table->construct_cell('<a href="index.php?module=config/menumanager&action=disable&id=' . $menuRow['id'] . '">'.($menuRow['disable'] ? '<font color="#FF0000">' . $lang->menumanager_disabled . '</font>' : $lang->menumanager_enabled) . '</a>', array('class' => 'align_center'));
to:
$table->construct_cell('<a href="index.php?module=config-menumanager&action=disable&id=' . $menuRow['id'] . '">'.($menuRow['disable'] ? '<font color="#FF0000">' . $lang->menumanager_disabled . '</font>' : $lang->menumanager_enabled) . '</a>', array('class' => 'align_center'));
Line 272:
$table->construct_cell('<a href="index.php?module=config/menumanager&action=edit&id=' . $menuRow['id'] . '">' . $lang->menumanager_edit . '</a>');
to:
$table->construct_cell('<a href="index.php?module=config-menumanager&action=edit&id=' . $menuRow['id'] . '">' . $lang->menumanager_edit . '</a>');
Line 338:
admin_redirect("index.php?module=config/menumanager");
to:
admin_redirect("index.php?module=config-menumanager");
Line 351:
$form = new Form("index.php?module=config/menumanager&action=add2", "post");
to:
$form = new Form("index.php?module=config-menumanager&action=add2", "post");
Line 433:
admin_redirect("index.php?module=config/menumanager");
to:
admin_redirect("index.php?module=config-menumanager");
Line 446:
$form = new Form("index.php?module=config/menumanager&action=edit2", "post");
to:
$form = new Form("index.php?module=config-menumanager&action=edit2", "post");
Line 503:
admin_redirect("index.php?module=config/menumanager");
to:
admin_redirect("index.php?module=config-menumanager");
Line 510:
admin_redirect("index.php?module=config/menumanager");
to:
admin_redirect("index.php?module=config-menumanager");
Line 531:
admin_redirect("index.php?module=config/menumanager");
to:
admin_redirect("index.php?module=config-menumanager");
So... yeah, not many edits
But you'll have to make them for it to work, as the author of the plugin didn't.