When you are checking out. You will see a page asking for your card details etc. Below there is a box. Put the code there.Also, those who got the code, have you been buying regularly and is the code the same as the one posted?
I suggest you take some time and browse through the earlier pages in this thread. You will be able to see what the different brands offered during a particular season. Even though you won't see pictures/details of all, the handful that caught the attention of the Ballers would have been posted/discussed. Just post if you have any doubts whatsoever.
^ 10sei seems all over the place. It depends. Some are Guidi. Those Jp stockists usually state the leather used in the description column. For instance, Antonioli does not, but you can check with them over email.