Act 1 - find Outpost of Amazons - should be 3rd town with waypoint. To make it more clear - second town Tran Athulua has two exits - one in the top left and one in bottom right. Take that bottom right, explore jungles until you find big temple (same design as original Diablo 2 act 3 Travincal). Explore that temple, get cube. Leave temple and go bottom right of the map. You will find Outpost of Amazons there with a waypoint. Explore oupost suroundings - you will find entrance to Mont Arnazeus.
Where to find Mont Arnazeus
Act 1 - find Outpost of Amazons - should be 3rd town with waypoint. To make it more clear - second town Tran Athulua has two exits - one in the top left and one in bottom right. Take that bottom right, explore jungles until you find big temple (same design as original Diablo 2 act 3 Travincal). Explore that temple, get cube. Leave temple and go bottom right of the map. You will find Outpost of Amazons there with a waypoint. Explore oupost suroundings - you will find entrance to Mont Arnazeus.