A couple of hours before you want to eat stir in the dry macaroni. You CAN put it in at the beginning, but it will grow and grow, especially if you let it cook all day, so if that's your plan, start with really tiny pasta, like tiny elbow mac or stars.
I sometimes just cook the pasta in a pan of water and stir it into the goulash 15 or 20 minutes before I serve it. That way, it's done perfectly--isn't mushy or "crunchy." Pasta can be very tricky in a slow-cooker.