Milker is from Man Workbench, go bring the female MC to the southeast beach with the tent at midday, there are already maps posted for that.
Npc not working on Breeding Farms can be a bunch of things. Make sure they have their area set by clicking them, and hit "Move", raise their territory to encompass all the Breeding Farms/Milkers/etc stuff. Set their location so that all the breeding stuff, their sleeping area, food, and water are all in range of their territory. Make sure to also place a Harvest Box within it, that's where they drop off materials.
The Milker will drop mother's milk inside the machine itself, but the breeding farm requires the Harvest Box. Of course the Milker also requires that the girl previously gave birth, I think abortions don't count, so make sure you have a midwife/bed to give birth or the chance to succeed on pregnancy is terrible. You can always banish the child if you don't want them. It's recommended you make the Pregnancy Promotion Items before you start trying to impregnate girls (From campfire). It's an equipment, not a consumable, so it lasts forever. Impregnation chance is abysmal without that.
For those unaware, a single seed can soon turn into dozens of seeds. Don't take the fruit/vegetable out of the plot immediately, it will eventually turn into 4 seeds. It's much faster with a bunch of girls working the farms. So you only need one Soybean Seed to start making a ton of Pregnancy Promotion Items for all your girls. The beehives are a bit more obnoxious since I don't think there's any way to farm them, just gotta wander around and hope for good rng.
