In the movie, sometimes they show a view of the spaceships flying over the town (very fake looking) and they look like flying saucers. But in the scene where they show people running into the spaceship, the spaceship looks like a building, and is perfectly stuck to the ground. There is even a flat ladder stuck to the side of it!
Well, Plan 9 was a movie constructed from a lot of stock film footage, so it's not surprising that stuff doesn't match up. I think there's another scene where you see a police cruiser (stock footage) racing down a road, and when the actors get out of the car, it's a totally different make and model car!