There's only one point which you get into the mill. That's when you have an appointment with Vesna. Next to that appointment the mill will be closed for you.
The Mill is easy enough to missLook for stairway down just before approching the gate to the city**SPOILER**The link is a map of the outskirts so don't click if you don't want a spoiler.http://guides.gamepressure.com/thewitcher/gfx/word/68870984.jpg# 57 is the Mill locationBe well
Go to the village, where also the reverend and the chapel is. Then go straight ahead to the Maribor gate. Before the gate, take the steps on the right. Follow the path next to the water and you will see the mill on your rightside. Be aware that in the evening the place is not so safety.