The Boston Common Softball Field - on the right hand side of Charles Street near the intersection of Charles Street and Boylston Street.

From the Masspike (I 90)
Take Masspike Eastbound
Take the Prudential Exit
Stay right for Copley Place
Continue on Stuart Street and take left on Charles St South which will turn into Charles St after the first traffic light.

From the Southeast Expressway (I 93/RT. 3 North)
Take exit 20 toward I-90, Airport & South Station
Take a left toward South Station
At the light Turn LEFT (West) onto Kneeland St
Kneeland St becomes Stuart St
Turn RIGHT (North) onto [Charles St South]-follow it to Charles St [1 block]

From 93 South
Take exit 26 toward Storrow Drive-North Station
Follow Storrow Drive Heading Westbound:
Take the Backbay/Copley Square exit (be sure to stay in the left lane, since it's a left lane exit)
Once off the exit, take a left at light
Take first right onto Arlington Street
Proceed through set of lights
At second light, take a left onto Boylston Street
Go straight through lights
At second light, take a left onto Charles Street

From Storrow Drive
Heading Eastbound:
Take the Downtown Boston exit (right lane exit)
Follow the direction above from westbound