You can park your car in front of the chapel. Then pass the chapel on the left side. You see there billboards with information about the menhir. Walk along and through a narrow passage you will find a small path that leads directly to the menhir on the top of the hill.
By the way. This menhir is a replica. The original is to be seen in the museum of arts and archaeology in the centre of the city of Luxembourg.
Something is not right. This message is just to keep things from messing up down the road