How do you get to the other portals in amity square in Pokemon platinum I can't get to the one on the platform with no stairs and there is an item there.?

If you take one step into those buildings, you will also notice you can take another step either forward, left, or right. Try going in different directions to get to other portals.

The item is a spooky plate. So if you still want it you have to go in the right side of the amity square. Go in then pass the first cave and go to the next. Turn left in the 2nd cave. Then turn right in the cave you just came out of. The guy there gives you kornn berries. Go back to the cav and turn right in it. Then your there. To leave just go strait back.