Head north from Campbell River towards Sayward. Drive past Roberts Lake until you hit the Big Tree Main on your left (gravel logging road; there will probably be a pick-up or two parked at the highway junction).
Follow this road, across the Salmon River bridge, across another little bridge and soon after there will be a main road on your right. Turn right onto the Salmon River Main and follow the road until you come to a metal gate at a secondary road (on your right).
This is the entrance to the campsite; the gate should be unlocked.