SJSU Blethen Field is on 10th street north of SJ Municipal stadium.  The SJSU Softball and baseball fields on on the other side of the Bud Winter Field Track right on 10th  & Humboldt Sts. (north of Alma St.) 

From 280, take the 10th/11th st. exit and go south (One way) on 10th.  Stay on 10th past Keyes and look for the entrance at Humboldt St. (only goes left) or for another entrance further down 10th by the Track (Bud Winter Track) on the left, in the long block before Alma Avenue.

If you hit Alma and 10th turn around and go back toward Humboldt.  If the gate on 10th is closed near the track then go back on 10th and turn right on Humboldt St.
There may be a game at Spartan Stadium.  If so then they will charge to park in that lot, so just tell the attendants that you're there for the baseball game at Blethen Field, or park on the street.