Thanks for reporting -- I found a bug in the "registration disabled" message page. You can fix it by applying the patch at http://pkp.sfu.ca/bugzilla/show_bug.cgi?id=8409
As that suggests, rather than the "registration" link disappearing entirely, the user is being sent to a page explaining that registration is currently disabled. Check Journal Setup to see if registration for reviewers, readers, and authors are available -- if none, and if you're looking at a journal-specific page, the registration link should be hidden. If one is available, or if you're looking at a site-wide page, the link will be visible.
Public Knowledge Project Team