to ANYONE who knows ANYTHING about a using a multitrack.....

Hello to all....I have a urgent problem. This sunday I will be having a musician come in to put down some vocals onto an accoustic guitar track that he has.....Problem being that I just recently moved and do not remember how to wire up so that he can hear both the track and his own vocals in the headphones. I am using a Yamaha MT400 multitrack. I run from my pre amp into the chanel 1 input and then out of the input into the back of my Audiophile external soundbox.....So I guess what I am asking is, where should i run the soundbox intot he board and what settings should that channel be on....as in "Line/MIc, Mix/Cue...etc. Also on a last note...is it best to have the channel that my mic is running into set on Line or MIc?
Thanks to everyone...it is so very appreciated and so so extremly helpful. Jimmy