The mill is located in North wheat field by the farmer who sells hops, at one point miss charm will ask you for a gold love story, any gold story will do, after that she'll ask you to talk to the vegetable farmer about his daughter, after talking to the farmer his son will tell you to fix his uncle's mill. achievement will unlock upon fixing mill.
Edit: calculation papers are needed and obtained from astronomer, I forgot to mention those.
5 complex parts, 6 wooden planks, and calculation papers fix the mill
Many of the quests in this game are intertwined and take some time to complete, so if it seems you cannot advance a certain quest try advancing different ones until the paths cross.