You can choose your papers once you have accepted your Offer of Place and enrolment has opened for the year (this usually happens in the last three months of the previous year). When enrolment opens, you will see a red button in MyWaikato under 'Qualifications and Papers' which says 'Complete enrolment'.
Select this button to complete your enrolment, working through each of the steps in order. Choosing your papers is the last step.
If no button is showing yet, you can relax for now. You will receive an email to let you know when enrolment has opened, so you can log in and complete your enrolment.
You need to choose all your papers by the Friday before your semester begins. If you need to change papers, there are deadlines for this too.