Solved.
Reading the code in plugins/system/t3/includes/admin/megamenu.php

$query->where('a.id in (1,2,3) or a.title = ' . $db->quote('Guest')); //we only support Public, Registered, Special, Guest

I noticed that for displaying "guest" ACL items in menu, I MUST create a new access level and a new user group called "Guest". Other names are not allowed: it's not trivial

Greetings

Hi Daniele,

Great to see it work for you now. I just want to add some more info related to the topic and hope it help you.

It likely belongs to Joomla core. Megamenu just supports the way menu items are displayed. For access level, it's operated by Joomla. If you want to hide Menu item A for visitor, just set "Registered" in the "Access" field. Once done, in front-page, only registered user (user login to your site) will see the menu item A. In Megamenu setting panel, you can see the access setting but it's for filtering purpose and build megamenu for each menu access level. For example, you can build megamenu for Registered access menu items that is different from Public access menu items.

Gardner.

Hi Daniele,

Great to see it work for you now. I just want to add some more info related to the topic and hope it help you.

It likely belongs to Joomla core. Megamenu just supports the way menu items are displayed. For access level, it's operated by Joomla. If you want to hide Menu item A for visitor, just set "Registered" in the "Access" field. Once done, in front-page, only registered user (user login to your site) will see the menu item A. In Megamenu setting panel, you can see the access setting but it's for filtering purpose and build megamenu for each menu access level. For example, you can build megamenu for Registered access menu items that is different from Public access menu items.

Gardner.

i'm having a problem with this. i hava guest level and a registered level for access. mega menu shows those two fine. But i do have another level called "Team member" which has access to more things than registered users. But the team member acccess control doesn't show up for megamenu even though it shows up for joomla.

Hi again,

Please pm me your site url, admin account, I need to check the issue on your site, the user group setting, megamenu configuration, etc to figure out the problem.

Gardner.