DH and I did this on our recent trip...you enter the same way as everyone else (through the main entrance) but when you get to the top of the hill, there's ropes set up to move people to the center/left and other people to the right - you show your ticket from the dining plan and they shuffle you right. So you still get the reserved seating the same as you did before, but now you just enter the same way as everyone else.
One weird thing for us was that our waitress told us to be there an hour and a half to 45 minutes early...I thought it was weird because the whole point is to not have to get there that soon! (I know you have to be there 30 minutes before - but 90??) We got there about 45 minutes early and had great seats, but it was October and not a crowded night at the park.