According to the football ground guide the best bet is to get the train from Manchester United Football Ground Station to Piccadilly which takes about 10 minutes and the entrance is immediately outside the away end.
Don't think it makes much sense for Dunk to deliberately get booked that would leave him only 5 bookings away from a 3 match ban and potentially missing very important games towards the end of the season. We really need him to go a number of games without a booking