All you have to do is once you get to the front of the ride, ask the TM who is standing in front of the chained off line on the far right, if you could take a castle tour, you can either go by yourself or you can wait for one of the TMs to guide you. I usually go by myself however almost everytime i've went I have seen other groups taking the tour with a TM as their guide. It is not to be missed, since I personaly think the hall of portraits is ten times better, and you can take the time to watch the entire preshow in the datda classroom which unless your lucky is kind of hard to do in the regular line.