This treasure is a bit difficult to find (it's meant to be, too). Basically, you should be able to get to the exact location if you follow the small red flag signs Osmund placed at most junctions.
But if you can't (or if you're tired of it), see the Walkthrough text for a detailed description: Press F1, select Walkthrough, select chapter Usipians' Refuge. (Since the walkthrough is made of seperate chapters, this won't spoil the rest of the game for you.)
In this walkthough text you'll find the exact coordinates (MapNo,X,Y) of the way you need to go. To find out your current coordinates, press "P" three times: Now if you're e.g. at 40,70,70 and the Walkthrough tells you to go to 40,70,65, then simply walk up (North) five fields - and so on. The treasure can be reached via 40,29,86 / 40,25,48 / 40,73,21 / 40,85,18 / 40,74,43 / 40,102,42 / 40,113,28 / 40,116,40 / 40,121,56; and it's buried at 40,121,54. See Walkthrough text for detailed description. This way, you should be able to find the location without any effort...