I could only find it after Dairou sprung old Shujinko out of jail and then I had to beat Hotaru. Then after that, I had no problem finding it, but I think you have to meditate in the area some for it to appear. I think it's like H1? Something like that. It's somewhere along those outer walkways.

It is at H-1 in the early morning. You have to make time speed up and look around while you are doing it. You see the treasure box appear.

if you look at the map of orderrealm, you will see how the level is primarily made up of concentric rings. on the outermost ring you will see 4 "arch-shaped" walkways, 1 in each corner of the map. in the center of each archway is a walkway going straight out. to get hotaro's key, go to the upper-right archway (it is the archway on the side where goldtown and redtown are bordering) and walk out as on the straight walkway in the centre of the arch leading away from the map.
also, on the other archways are treasure chests :D
hope this helps