If you take the road that goes north from Dunvegan, past the castle, and follow it to its end at Claigan, you'll find a small car park there. The souterrain is just a short walk from the car park, through a gate and up a cart track to NG238539. Just after the track bends hard right, it is about 20m off the track to the left. It is not too difficult to find, although you may need to scout around for a bit to see the entrance.
You will readily get a sense of the area of the site because it has that characteristic short grass and lumpy surface that often indicates prehistoric constructions. The entrance is towards the far side of this area. It is a small and partly overgrown hole in the ground, but on closer examination the lintel supporting the roof is obvious.