At MIT, at the main athletic fields. At the first light, make a right at Alphonsus and go up the hill. It will turn to the left after you pass Hillside. Then you make your first right on Judge Street. Directly in front of you at that point is the park, Parker Hill Playfield.
Parking is usually really easy.
You can also get there by the T on either the E-green line, and Roxbury Crossing on the Orange line. At Chelmsford High School fields. From 128 take route 3 North. Go 100 yards. We'll either be on the field inside the track to the left or on the open fields past the softball fields on the right.