the main quest isnt marked do u know how to mark it on the 360
No idea how the 360 version work out, but on pc version, main quests location are usually marked on the map itself. We can manually add own marker on the map as well.
As for Sassan's grave, its located at the northern part of the map of sentinel island, which is very close to the ghost of the fisherman/sailor, by the sea shore.