Is this a typo? It is listed as a “new proficiency” in the Dwarven Machinist class description, yet no description of the proficiency is to be found in the Players Companion. It is also not in the basic ACKS book.
The description of Mechanical Engineering is on p. 96 of the PC, in the custom powers section.
It should probably be repeated in the new proficiencies chapter (p. 153).