After following the explanation on http://docs.moodle.org/en/admin/mnet/access_control ... I was at point 3.4, in section 3 ("Peer to Peer Network"). After clicking on the peer site's name, I get the error message "RPC enrol/mnet/available_courses:ERROR 0:" while trying to display (what I think would be) the courses available on the peer site.
Incidentally, on that same page, under the "courses" column, the cell displays "? - enrol". I'm not too sure of what that means either.