I've just been to this site and it can be a bit tricky to find. The exact co-ordinates are: N: 46.66380 E: 1.25701. It is in amongst a cluster of trees on a hill in a field. The way to the Dolmen itself is signposted from the Route De Ciron. Its a nice place, very serene and peaceful.
Something is not right. This message is just to keep things from messing up down the road