The item doesn't unlock in HG/SS. There is no way to open it. In HG/SS it was a filler item that never got finished, like many things from the past.
For B/W All you have to do is take it to Mr. Locke and he unlocks it. There aren't any specific requirements to unlock it other than it has to be Transfered over from HG/SS through the Download thing...I forgot the name but its how you transfer Celebi and the Shiny Beasts to unlock the Zorua/Zoroark events. If you don't do the transfer that way then the events don't work.