Early in the game after Link first visits Hyrule Castle, Mack and the rest of the Carpenters can be found in North Hyrule Field. If Link tries to speak with Mack, he simply states that he has no time to talk as the boss has him working his fingers to the bone.
For much of the game, Mack can be found cutting wood within a small hut at the southwest portion of Hyrule Town. If Link tries to speak with him, Mack will say that he is busy and that the boss always makes sure that they've got their noses to the ol' grindstone.
Link is able to fuse Green Kinstones at various points in the game with Mack. As it is a Green Kinstone fusion, it is not set to a specific treasure chest, but instead, one of the many possible treasure chests. There is one random Red Kinstone within the game that opens up a location in the Hyrule Castle Garden leading to a Piece of Heart, as well as a random Blue Kinstone that opens up a tree in North Hyrule Field leading to a Fairy Fountain. Mack is one of the many characters in which Link can make this Kinstone fusion with.