From Boston:

Take Huntington Avenue away from Boston to the Jamaica Way
Follow the Jamaica Way until it turns into the Arborway
Continue to follow signs for Route 203 East
Go through Mattapan toward Hyde Park. You will come to a Mobile Station on your right at Blue Hills Ave (Rt 28, but no signs). Turn Right.
Go 1.1 miles and Turn Right onto River Street (the sign for River Street is actually on the other side of the street, so keep a good eye open)
Go 0.8 miles and turn right onto Reedy Street and the field is at the end of the street

From Route 95:

Take Exit 14 to East Street/DEDHAM (follow signs at the rotary after getting off the highway)
Follow East Street 0.8 miles, and the street will come to a triangle.
Bear left at triangle, remaining on East Street. Continue on East Street another 0.5 miles.
Turn Right onto Sanderson Ave and go 1.4 miles
Sanderson turns into River Street
Follow River Street (River St. will bear 90 degrees to the left)
Pass Turtle Pond Parkway and Kelly Field on your left and continue another 2.1 miles on River Street
Turn left (but you will still be on River Street) and go another 0.6 miles
Turn left on Reedy Street and go 0.25 miles. Ross Field will be at the end of the street.