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, CA/T 909, has its own waymark). BM-203 MCC is located atop the corner of the concrete barrier preventing things from falling onto I-93.
This mark is actually unlabeled and unstamped. It is of the same shape as the other CA/T marks, but without a triangle or other identifying marks on it. However, at the time of discovery, there was writing on the concrete next to it indicating what it was. This writing read:
EL = 135.01
BM-203
MCC
EL is presumably an elevation, although in what units, I'm not sure. My GPSr gave an elevation of 16.71 meters, which doesn't readily translate into 135.01 in any units, but as far as I know, GPSr elevations are dubious in general. BM-203 is an identifier, I'm guessing, and MCC probably stands for Modern Continental Construction, a firm that has been doing much of the work on the Big Dig.