First, get yourself to the Alewife MBTA station. It is on the Red line. Take either the #62 or #76 from Alewife to the stop at the Minuteman statue in Lexington Center. This is the final stop before the two bus routes split. The trip takes about 23 minutes. These are the relevant busses:
When you get off the bus, cross the road towards the green, and then keep going until you reach the white church directly behind the statue. Walk down the driveway to the right of the church to get to the rear door, where you enter. This walk is only 3-5 minutes.
For further information on the MBTA system, see http://www.mbta.com.
Thanks to Nicholas Cheung for providing this information.