If it is a paid activity (like Fish are Friends or Surfs up with Chip Dale) then you can call ahead prior to your trip or when you get to Aulani.
If it is an activity that is free but limited space (like Stitchs Space goo or the Volcano eruption thing) then you need to register in the morning the day of the activity. Earlier the better, as it does fill up.
They open at 8 and if there was an activity my kids wanted to do then I lined up around ten till. Sometimes there were 10 people ahead of me and more quickly behind me, sometimes just a cpl of us
Also, if you want your kids to have lunch or dinner there ($8.50 ea) you also need to sign up in the morning. That is also limited space and had waitlists.