The castle show at IOA goes on about every 15 minutes or so from sundown to shortly before the park closes. Enter Hogsmeade from the Lost Continent area. The crowds start to build around the entrance to the Hogwarts Express. Just be patient and move forward with the crowd. It’s tough to explain how it works exactly. Just be patient and you’ll see the show.
For the Cinematics show at USO, watch from Central Park. The show is basically designed to be watched from there. It starts towards the end of the park hours for the day. Check their website for specific times as it changes depending on when you go. Depending on how crowded the park is that day, I suggest getting a spot about 30 minutes before showtime. There are various “levels” to watch the show from in Central Park. I’d go to the middle one or top one, but that is a matter of personal preference as the view from the whole area is good.