If you are not on a tight schedule, and do a little planning before hand, Boston's subway (called the "T", which stands for transit) is the easiest way for locals to get from the airport to the city. And for visitors who want to act like locals. The map is color coded and easy to use. (similar to the underground in London or the Metro in Paris, but in English) (Just find the right color, and find the right direction.)
http://www.mbta.com/traveling_t/schedules_subway.asp
You take the blue line from the airport, and then usually transfer to another color or two depending on where you are going. (Sorry I stayed with a friend near the MIT campus, but I know a lot of people stayed at the recommended hotel which was within walking distance to the interview. Looking at the other post I think it was the Hampton Inn.)
Just make sure you go to the correct campus. (Depending on which program you are applying to.) Directions should be in your interview packet.
The buses are also useful (for going to the interview from your hotel) if you catch them right, but you might consider splurging for a taxi or walking depending on where you stay. It will be cold, though. And for someone from Arizona, you will freeze and think you are going to die. Have chapstick and gloves, or keep your hands in your pockets. So if you can get the walking distance hotel, pay whatever price, it will be worth it if you are not used to the city of Boston. (But if you arrive the night before, have an adventure on the "T" to get to your hotel. You just need to find the closest "T" stop.)
Good luck!
