If you come from the south on E6, take road no 110 (Oldtidsveien) towards Fredrikstad.
From the north drive over Glomma at Sarpsborg, take road no 111 towards Fredrikstad, after a few hundred meters take the road (to the left) to Borge church, this road meets road no 110 (Oldtidsveien) after a few kilometers (at Borge school), take to the left again and you'll soon be there. There are two parking lots, the rings are closest to the second.
There are 3 areas of interest at Hunn. The stonerings are in the southern area. This is the english text from the information-board:
On the slope in front of you, you see one of the biggest and most magnificient clusters of stone rings in Norway. There are in all nine big rings of upright stones. Moreover, there are a number of burial cairns and circular cromlechs. Each ring consists of 12, 13 or 15 stones. In earlier times it was assumed that trials had been held in such sites, the defendant standing in the middle, and the judges seated around, one on each stone. Archeological research, however, has proved the ring sites to be burialgrounds, probably from the Pre-Roman Iron Age (500 BC - AD). Apart from some charcoal and burned bones, nothing has been found in the rings, which clearly identifies them as fire graves, where the deceased had got elaborately shaped monuments - instead of equipment and gifts in their graves...
The norwegian infoboard elaborates a bit. Some of the rings and cairns were excavated in 1950-53, and several fallen stones were raised. The stones in the rings were connected with packed stones. In the centre of some the rings were found a large flat stone or a "package" of stones. (C-14-datings of charcoal from a similar stonering in the middle site at Hunn dates it prior to, or between 520-280 BC.) The cairns seems to be younger, six stone-pearls and two bronze-buckles dated viking-age (800-1050) were found in two of them. In the 1970-ies a few rockcarvings were found in the wood to the east close to this site. Locally this site is known as tinget.