Maybe I'm wrong, but the impression I had is that, at many schools, the person who presides over admissions is simply called the "associate dean of admissions." I don't think there necessarily exists anyone higher up. The reason why he has the word "associate" in his title might simply be because he's in charge of admissions rather than in charge of the whole medical school.
Also, if you're going ot take ClockworkDoc's advice and contact the admissions office, I'd personally ask them where you should send "additional materials for your file." I'm not sure whether the phrase "letter of interest" is universally understood yet, and in any case it sounds kinda canned.