Useful addresses for navigation systems:
Brown Field: 175 Wheeler Road, Newton, MA 02459
South Fields: 141 Brandeis Road, Newton, MA 02459 (park anywhere around the HS complex).
Weeks Field, Oxford Road side (east of the fields): 107 Oxford Rd, Newton MA 02459 (anywhere on that block is good -- there's an alley to the fields)
Weeks Field, Paul St side (north of the fields):  66 Paul St, Newton, MA 02459
Weeks Field, Rowena Rd (south of the fields):  3 Rowena Rd, Newton, MA 02459

Click on the picture below for directions between fields from MapQuest.  (Note -- Google Maps and some others seems think that you can't take Parker street across Route 9.  This is not true -- Parker Street goes right over Route 9 in both directions!)