1-Hit the panels that say Star and Wars (just above graphic of Vader's head). This will open a hatch just below these panels that the ball will roll over if coming down from above but fall into if coming up towards it.
2-Sink the ball into this hatch. You will have to choose Scenes. Pick Scene 2.
3-Scene 2 is defeating a Stormtrooper who appears in that central area. He aims either left or right. You have to hit him from behind. This is tricky because the hatch is open still, so you can't fire the ball straight from the flippers. You have to hit the loop that goes around the Star Wars panels. When your ball is above the upper flippers there, it will pause a moment. The moment the ball starts moving again, hit that flipper and it should hit the Stormtrooper. He neds 3 or 4 hits.
4-Once this is done you move on to the AT-AT finally. This is also a checkpoint so that if you fail to down the At-AT, the next time you start Scene 2 you won't have to do the Stormtrooper again this game. To down the AT-AT, you need to go around the loop behind the Star and Wars panels 5 times, alternating directions (clockwise, counter-clockwise, etc) I've not done it yet but I believe that's all there is to it.
Best of luck