In order to find Ancient's Way, you first must get to the Frozen Tundra. You can find its entrance from the Glacial Trail. Teleport there if you have a Waypoint or travel through the Crystalline Passage. From the Frozen Tundra (make sure to set a Waypoint) you'll find the entrance to The Ancients' Way (set Waypoint there as well). The entrance to Arreat Summit is inside.