Luigi's Mansion

IIRC, the thorn Doors only open after you have recieved a key from the final boss of a Part. If you have progressed through the first thorn door, then you should be in Part Two. If you have finished with everything else, proceed to the Dining Room, then to the Kitchen, then to the Backyard. After you defeat the mini-boss here (it's a dog) grab some Water Element, and water the patch of dirt. Come back later in both Part 3 and 4 and to water it, and you will be rewarded with a mound of money, but only if you remembered to water it all three times.

Now, turn your attention to the dog house in the area. Examine it with your Game Boy Horror, and you will be warped to the Graveyard. Here you will fight the boss of Part Two, and victory will grant you the key you need.

Hope that helps.