This is a difficult description to write because the mark is located at the intersection of two streets on a very new portion of the Big Dig which, to the best of my knowledge (and research on several official web sites), are unnamed. But here goes.
To get to the mark, the most sensible way is to start out at the intersection of Lincoln and Kneeland Streets in Chinatown. Go west along Kneeland Street for one block. Turn left at the intersection (which will have a Teradyne building and a building with a "Welcome To Chinadown" sign on it). You will now be walking on the eastern sidewalk of a frontage road that runs between Hudson St and I-93. Note that at the time of this writing, the sidewalk was sort of blocked by a construction barrier, but not fully (which I took as an invitation to continue).
Continuing up this frontage road (which, by the way, after going over I-90, turns into Albany St), you will come to an intersection with a stoplight. The road that intersects the frontage road goes over I-93 and into South Station, for what it's worth, but you should cross it at this point. You will then be at the southeast corner of the intersection.
There are two marks located here (the other, BM-203 MCC, has its own waymark). CA/T 909 is embedded in the sidewalk, approximately 8 inches west of the end of a blue railing preventing people from falling on I-93.