There is no online pre-reg for Regionals, only Nationals. Like Brett said, click on “Information” to the right of the National event and there will be a link to online registration for that event.
EDIT: I just looked and the pre-reg links don’t seem to be working for the races this weekend. It may be that online pre-reg is closed at this point. Upcoming Nationals have “TBA” where the “Information” link would be. Guessing that the Info link and pre-reg links will be active for these as we get closer to the races.