I live near this site. I make no judgement here, but for the information of other visitors to this site, where Carl parked was on private property, and the track leading to the stone is also privately owned, with no public right of way.
Not being clever (speaking as one who frequently sails past sites both on foot and in the car) but it really was easy to find being signposted off the road where the TMA directions make it sound a lot harder. Never mind, I'm sure you'll get back there one day. In the meantime, enjoy Lewis and Orkney.