Thanks, this worked !
For the others in the thread :
I downloaded the ps_mbo module from the prestashop addons website (https://addons.prestashop.com).
This module adds a new function to your back-office, a new page that appears in the menu under modules > marketplace. This admin page gives you access to the addons marketplace, kinda like on the addons website, except that on this page I was finally able to find the ps_accounts module !
I still don't understand why the ps_accounts module is accessible through the MBO page and not directly from the addons website, but at least this resolved my problem.