From San Antonio (East on IH 10) :

Take exit 604. Before you get to the exit ramp you will see the blue ExploreUSA sign on your left. You'll have to loop around and come back on the west bound access road.

From IH 10 (traveling West) :

Cross the Guadalupe River and take exit 604. Go through the stop sign and we're approximately a half mile ahead on the right.