How to reach Drunder, Fortress of Zek
You can find the stone to click that will zone you into Plane of Tactics inside the cave that's in central Plane of Tranquility. When you run into the cave, the stone to PoTactics is the first one on the right. Unless it's some time after the Planes of Power expansion pack you'll need to have some PoP flags in order to enter this zone.
If you still need help finding this location, take a gander at my map below.