I didn't find this but I didn't really get a chance with some ill-luck. Bruno Marc's description in Dolmens et Menhirs en Languedoc et Roussillon says it's about 400m to the west of Flouirac farm with private access. (There's also the Menhir de la Caucalière near it.)
When I arrived at the farm there was nobody about to ask even after knocking at the door. So I was was just going to write a note to put on the windscreen of my car & set off anyway, when an elderly woman called from the upstairs window. She spoke unusually perfect English but what she said, I didn't want to hear. She claimed that there was no dolmen other than Dolmen de Flouirac back down on the road!
I'm not sure if this was a blatant lie, but there wasn't much I could do. It did occur to me to drive back down to the road, park, and then sneak back on foot. But I didn't really have time & wasn't comfortable enough with that degree of subterfuge.
I've been unable to find any photos or confirmation that the site still exists, though a quick Google search shows it is mentioned on some geocaching websites - suggesting it's still there....
Dispirited by 2 failures in a row & pressed for time, I went back down to the road & after another vague scan for Dolmen de Flouirac, I started the long drive back to Arles.