Yes the Dorsey Hall address is the mission office. I would suggest to write to that address if you don't write your friend very often. If you do write to her/him send it to the apartment that address until your friend tells you that they might be transferred. It's always easier to get packages right at the appartment but it's alright to send it to the mission office. There are always suprise transfers. I myself was suprised to get a telephone call my very last month, to tell me that we were going to close my area and both be transfered the very next day. Most missionaries live in Apartments. There are few that live in houses with members.