there's a few criteria in order to get Z seats.

If I remember correctly:

1) you have to buy the ticket more than 30 days in advance
2) the length of travel has to be a week or longer
3) it doesn't allow stopovers

If you're not seeing Z seats online, call Reservations and see if they can bring them up for you.

It may also end up being that there are no Z fares being offered for the period in which you want to fly, or (more likely with the smaller A332's flying TransPac operations) they are already sold out.