Where is the trophy garden on platinum?

Head down from Hearthome City onto Route 212. On your right, you should see an enclosed space with an opening on the bottom. Go through the opening and head up into the House. Once you're in the mansion, head straight up and you'll find yourself in the trophy garden. If you speak to the owner of the mansion who's found in one of the rooms on the right, he will talk about a unique Pokemon that you can find in the trophy garden for that day.