Oh boy... let's see^^
I'm not too sure on that cause it's been a while but let's try^^
Travel to the Myrad(hope it's the right word, haven't played in english yet^^) tower. Follow the road north, I believe and somewhere should be a road to the east. Follow that, you go through some ruin where I don't remember the name anymore (could be Thalgard or something like that). There should be some stairs down the mountain towards the shore a little above the ruin where you get the cube.
Hope it's not too confusing and hope you find your way down there.