I've uploaded this to my development one and it works perfectly, so the only thing I can think of is that there is something else affecting it. Without me taking a closer look at your forum setup, edit the plugin file, and change:
...to...
Let me know how that goes...
Also, just thought, please make sure you're editing the correct user(s). They must be apart of the "Moderators" group (or just have access to the ModCP). This will not affect Super Moderators or Administrators.
$plugins->add_hook("modcp_start", "modoptions_perms");
$plugins->add_hook("admin_user_users_edit", "modoptions_hooker");
$plugins->add_hook("admin_user_users_edit_commit", "modoptions_commit");
...to...
$plugins->add_hook("modcp_start", "modoptions_perms", 1);
$plugins->add_hook("admin_user_users_edit", "modoptions_hooker", 1);
$plugins->add_hook("admin_user_users_edit_commit", "modoptions_commit", 1);
Let me know how that goes...
Also, just thought, please make sure you're editing the correct user(s). They must be apart of the "Moderators" group (or just have access to the ModCP). This will not affect Super Moderators or Administrators.