Well, actually, it is an useable item. You use it once, you can't equip it. I don't know much about details, but the higher the HIT (item's and not sure character's) the better chance to steal. Thieves have greater chance and LVL also helps.
But, again, its a non-equippable item and you must use it in battle to steal (the character which uses it loses its turn)

you use it like a item

Yeah, it's a consumable... and it's sold in the consumable shop, which kinda hints on that as well :P

Yeah, thanks to all who answered! It was a silly thing for me to ask but I was quite lost :P