Did you have a good affection with beast enemy like werewolf, wolbauks etc.? I think it's one of the condition that acquire for this event.
I not actually sure about this, but I'll suggest you to try this method. Go to the forest nearby you hut and check nearby area (daytime) until you encounter with wolf that two choices will be appear, accept your position or defend yourself. Then, choose 'accept your position' or 'defend yourself' and submit. Try it several time (just for sure, do it more than 10 times) and go to talk with Roghak. The milking event should appear to be selected.
If it still doesn't appear, please try this method again one more time.