In general you should get the detector from the event, and the field map from the detector. If you only want the Z field, and you are prepared to assume the field is constant (since we don't have a detailed field map that is probably a good approximation) you can use:

Hep3Vector ip = new BasicHep3Vector(); 
double zField = event.getDetector().getFieldMap().getField(ip).z();

or more fully

import hep.physics.vec.BasicHep3Vector;
import hep.physics.vec.Hep3Vector;
import org.lcsim.event.EventHeader;
import org.lcsim.util.Driver;

public class FieldDriver extends Driver
{
   private static final Hep3Vector ip = new BasicHep3Vector();

   public void process(EventHeader event)
   {  
      double zField = event.getDetector().getFieldMap().getField(ip).z();
   } 
}

For more information see FieldMap