You can drive directly to the Bogcoup junction, taking the estate road from NJ627422. I found Drewbhoy's description confusing, perhaps because there is now a sizeable plantation of young conifirs screening the mature forest.
I followed the track (on foot) northwards, just past the edge of the field and through a gate on the left. This led to a tunnel of debranched trees that followed the field edge northwestwards. At the end of the tunnel, follow the field edge for a short distance (southwest) and, after just 50 metres or so, look out for a small plantation of much younger trees in a clearing to your right. Walk straight through the plantation for a couple of minutes until you arrive at a fence, with the wide forestry road beyond.
Cross the fence and follow the road leftward for 100 metres. The road now turns sharply left at right angles, following the plantation boundary. Continue for another 100 metres, and you will see the remains of Frendraught Stone Circle on the very edge of the forest on your right hand side.
An alternative path starts in the village of Forgue. Take the road signposted to Forgue School and park in the village hall car park. Walk up to the school where the road continues as a forest track. After 2 km, this track takes you to the same gate as mentioned above.
One bonus of this route is that you can make a short diversion along the way to visit Raich Stone Circle.
It is best to ask permission from the Frendraught House secretary before looking as shooting parties are often in the area. The chequered history of the house is well worth discovering.
From the office climb the hill until the road ends at its Bogcup junction. Turn right, walk 500 yards, turn right again. Keep going downhill and the path will gradually veer north into the woods. The remains of the ruined circle will come into view as the track leads straight to the recumbent. This has been battered, weathered, cracked and most likely moved but defiantly it remains along with it's fallen flankers. About 50 yards a stone from the northern arc of the circle can be seen. Unfortunately nothing else can be seen as the vegetation covers it although there is plenty cairn material lying about. Take good boots.
As said elsewhere when making enquiries it was obvious that the secretary and other staff didn't have a clue about the existance of this circle.
The RCAHMS record linked to below suggests that local people once knew the site as "The Covenanters Preaching Stones" (exact punctuation up to you I think) - the Covenanters, for historically ignorant sassenachs like myself, being 17th century Scottish Presbyterians.
But I wonder whether the stone(s) here is also the one in the following story:
One [tradition still floating about in the district] relates how, on one occasion, a brawny Highlander had occasion to pass the house [of Frendraught]. He was observed by the lady, who remarked to her husband that he was the prettiest man she had ever seen. This remark led to a quarrel, and her husband, filled with jealous anger, is said to have rushed from the house and followed the unsuspecting Highlander, whom he slew, and buried under a large stone near the hill top.
It seems likely, does it not. ??
From 'Some stone circles in central Aberdeenshire' by James Ritchie, PSAS 1917, volume 51, p30-47.