Greenfield exit, 2nd or 3rd street on left, go until you hit 13th and make a right. Park is on your left.
Greenfield is just north of King City, about 50 miles south of Salinas. Take the 101 south from Salinas, and exit Oak - heading west. You'll cross a little "downtown" area at El Camino Real, and continue on Oak for about 1 mile past 12th Street. You'll pass a school on your left, before you hit Patriot Park. The skatepark is in that park.