See The Great Cause 1) cache page (Click Here) for the cartridge and the first set of instructions. You must have completed the tasks required for The Great Cause 5) in order to complete the tasks required for this cache.
In order to sign the Edinburgh-Northampton Treaty you first must have acquired 200 Kudos points in order to have gained sufficient support amongst the clans. Once you have completed this task the location of the Treaty cache will appear on your map.
Details of how to acquire Kudos points are given in The Great Cause 3.
Download the Cartridge Here