What makes the LEAVE PROJECT being shown in the absences screen?

On a new installation, a hidden project "PROJECT - LEAVE PERIOD" is created in the "project" table.
Whatever I try to do, with "admin" user I can't see this project and use it in the "absences" screen.

What I tried :
- Changed "admin" to a resource, employee and leaves manager (checkboxes in the "resource" screen)
- Created a new leave type "Congé payé"

After this, I can see the project and activity in the "timesheet" screen.
But in the "absences" screen I stil get a message "Add some activities on some administrative projects".

I tried to read the documentation, but it explains how to use the module, not how to configure it...

Any suggestion to help me?