I can’t really improve on Julian’s directions to Adam and Eve (the ones he gives in the Modern Antiquarian book), so here they are:
At Beckhampton roundabout, take the A361 to Avebury and Swindon. After 200 yards , turn immediately left at ‘60s farm cottages and take the road straight on. Adam & Eve are in the field ahead.
As far as I can see there is no public access to the stones, but if there aren’t any crops in the field you could chance it and take a closer look.