In order to unlock open chat, you need to log in to your account on this website, than click "My Account"in the top right corner of the screen. From there, you need to enter your master password if you set it (the button is located in the top right hand corner of the new tabs that opened up in summary). This proves you have the administrator's rights to your account, as it were.
From there, click on the parental controls tab. This is where you adjust settings like gifting rights, level of speech with true friend codes and most importantly in this case, open chat.
Assuming you have an active membership, are over 18 andhave made a previous credit card purchase, you can simply click on the check box under On to turn on Open Chat.
I hope this helps in your quest to communicate more freely with your fellow pirates :).