Cerrig Meibion Arthur also known as The Stones of the Sons of Arthur and Cwm Garw are easily accessible along a farm track and is a ten minute walk (they can be seen from the road). You can park on the open moor (make sure it's not wet) - full details can be found here:

http://www.megalithic.co.uk/article.php?sid=1448

Some research of the areawill pay dividends as you won't want to miss out on the other gems!

You will need the Ordnance Survey Explorer OL35 map which is excellent and double sided.

For other great sites in the area get hold of a copy of the forthcoming White Dragon magazine if you can as I believe there is an article on the Preselis in that too (;-)).

Hope this helps and have a great visit.

Grendel