First I'm going to assume that you have your code for the Destrier Beret and have entered it into your Square Enix Account Management page. That is something that is done online, outside of the game.
Once that is done, chose which character you want the Destrier Beret to go to via the Account Management page. Log into FFXI, pull up that character and go find one of the Festival Mogs.
If you are in San d'Oria, exit Port Sandy and the Festival Mog is the one on the right just before you go down the stairs. Here is the POS for all the Festival Mogs.
Port Bastok (J-13) Port San d'Oria (J-10) Windurst Walls (C-14)
I did find a handy guide on Reddit for just in case you do not have or have lost your activation code. Follow the steps below and hopefully SE will send you a new code with no problems. Skyfiredancer (talk) 22:10, April 5, 2017 (UTC)
CarbunkleFlux 1 point 1 year ago If you didn't get your codes, you'll need to put a ticket into Square Enix support, say you didn't get your codes for Destrier Beret/Chocobo Shirt and supply your game/purchase ID if possible. They gave me mine without any fuss when I did this
I just did this yesterday.
1. Log in to Square Enix Account Management.
2. Go Add a Service under FFXI as if you were adding a new game to your account.
3. Enter the code.
4. Select the character you want to send the item to (once per code).
Make sure you logout from in-game and then visit a Festive Moogle (http://ffxiclopedia.wikia.com/wiki/Festive_Moogle).