**In order to get Pinoch as a summon, you must have chosen the summoner class upon starting your game.** If you are not a summoner, Pinoch will not join your party and this achievement will be impossible to get on your current playthrough.
Pinoch is a summon that can be found in the Marionette Factory. While on the last screen of the dungeon before the boss, choose to head east past the arrows that lead north to the boss of this dungeon. You will spiral through a path that leads south. You'll know you're going the right way if a giant pack of Marionettes attack you (some what of a mini boss). After defeating this mob, keep following the path to see Pinoch standing by his lonesome.
All you need to do to get Pinoch to join you is to speak to him.