Left turn when Western Way dead ends onto Buena Vista.
If you follow the signs inside Disney, it will take you on a ramp to World Drive, then through the Magic Kingdom toll booth. Stay far right, then right turn just past the toll booth, then Ft. Wilderness is on the left.
You can also stay on Buena Vista, and take a left onto Community Dr, then another left (not sure of the road name), then Ft. Wilderness will be on the right. However, the entrance into Ft. Wilderness was designed to come in from the opposite direction, so it is a "very" hard right. Might be harder with a big rig.