The milk ducts *are* on the underside of the breast, below the nipples. They are sort of straight and a bit rigid. I think you found them. Now to do the "right things" with them! Run your fingers up and down them, with some degree of firmness. You'll get some moans when you've found them that way.

As for men, a FEW men are stimulated by their nipples, but the milk ducts generally do a lot more. They're similar to a woman's milk ducts, but they're less developed and smaller. So... she's got to "look" for them.