How do you bind an item to a quick button in Pokemon Black?
That's not entirely true... You registered items via the Select button in GSC, RSE, FRLG but not in DPPt. It has been the Y button ever since the main Pokemon series games have been on DS...
On DPPt, it has been only one key item that you could register at a time accessible via the Y button.
On HGSS, you could assign up to two key items, one accessible via Y but both accessible via the touch screen.
On BW and BW2, you could assign multiple items (including specific menus) by checking a box. Items that can be registered have a checkable box to their left, and menus that can be registered have a checkable box at the bottom of the touch screen. Once they are registered, they can be accessed by pressing Y and then navigating the list of items/menus you registered
On XY (I know it's not entirely on topic, but just mentioning it), you can register up to 4 key items, accessible by pressing Y and then Up, Down, Left or Right on either the circle or D-pad.
Abbreviations used:
- GSC: Gold/Silver/Crystal
- RSE: Ruby/Sapphire/Emerald
- FRLG: FireRed/LeafGreen
- DPPt: Diamond/Pearl/Platinum
- HGSS: HeartGold/SoulSilver
- BW: Black/White
- BW2: Black 2/White 2
From the items menu, register the item, and then use the Y button in game to use the item.