You place the odd keystone inside of it. Then go to the underground and meet 32 players. You can get spiritomb.
>Players refers to actual people playing the game, who appear in the Underground via DS Wireless Connection. Talking to the NPC vendors or traders will not count towards the total needed to summon Spiritomb.
It is important to note that one does not need to talk to 32 different players (it can be the same person 32 times, 32 different people, or a combination of both). However, one of the players may have to leave the main part of the Underground between interactions for each one to count. Entering and leaving a secret base also counts as having left the Underground. The number of players the player has talked to can be checked in the Underground by selecting the menu and viewing the Trainer Card. The number of people met is the number of players one has talked to.
Spiritomb can be summoned multiple times; however, the player will need to repeat the requirements: a new Odd Keystone to place and another 32 player characters to interact with. - Bulbapedia