Visited here today. You can park at Belmont Centre and walk up the castle drive (ask permission, it's a site used a lot by schools so there are often a lot of children about). The stone's on the left a couple of hundred metres up the path, just inside a field which had several horses in it today. Not much to look at, about a metre high with a slight tilt to the west.
I spoke to a couple of people in the area about it, but despite having searched the Statistical Accounts and a couple of other references I can't ascertain why it is thus named - it sounds almost Danish, and certainly other stones are named after battles with the Danes. This one, however, seems to have no such tale attached to it.
In a field on the other side of the house is another monument to a hero of that day, to the memory of the brave young Seward, who fell, slain on the spot by Macbeth. A stupendous stone marks the place; twelve feet high above ground, and eighteen feet and a half in girth in the thickest place. The quantity below the surface of the earth only two feet eight inches; the weight, on accurate computation, amounts to twenty tons; yet I have been assured that no stone of this species is to be found within twenty miles.
From 'A Tour In Scotland and Voyage to the Hebrides, 1772' by Thomas Pennant (1776).
You were looking at the wrong sources Mr Brand! Seward's Stone is presumably named for Seward, Earl of Northumbria, who MacBeth killed. Whether or not this stone or the nearby MacBeth's Stone actually have any links with the MacBeth story I don't know.