Hi @amandasramalho,
Unlike OJS, OMP doesn’t have a page with a list of all available presses; our reasoning here is that it’s less likely for multi-press installations to advertise their holdings than it would be for a multi-journal OJS installation (since the latter could be institutional, or a commercial publisher, etc). Feedback on this is welcome, but the easiest solution will be to either modify OMP to add an index page, or to maintain the list outside of OMP (e.g. as a static HTML page).
Regards,
Alec Smecher
Public Knowledge Project Team