If it's before 12:30 am, you can ride the T (subway). If it's after 12:30 am, taxi is probably your best bet. Public transportation shuts down late at night. Taxis will be floating outside all the terminals at Logan.
To get from the airport to Longwood:
Take the Silverline bus to South Station
Take the red line (inbound, towards Alewife) to Park Street
Switch to the green E line (outbound towards Heath Street)
Get off at Longwood Station (it's about 8 stops)
Walking down Longwood Avenue (perpendicular to Huntington, the street the subway travels down) will bring you to Vanderbilt Hall (dorm) on your right and the medical Quad on your left. At the far end of the medical quad is Gordon Hall, where you'll start off interview day.
Subway fare is $2.50. I'd recommend having a couple of singles, so you can insert money once on the bus. Transfers from the bus to red to green lines are free. It should be fairly straightforward. Just be careful to get on the E line (not B, C, or D), and even though there are signs pointing to the track where E trains technically should arrive at Park Street, sometimes the train can arrive on other tracks as well, so just look out for that. Here's a map of the Boston T system in case you wanted to look it over before:
Subway Map
