Very easy to find, once you've been to the castle and well worth a look for the distant views of it alone. The barrows themselves are all bowl types of various sizes. The common is split in two by a road, the castle is visible from the western side, the eastern half of the Purbeck hills can be seen in it's entirety from the barrows of the east side of the common.
One of the east side round barrows has obviously been excavated in the past, it has a large cleft in the top of it. Of the six barrows I could see, they appear to be in a row from west to east, parallel to the Purbeck hills to the north of them.