The tree is just decoration, its not a physical location you can travel to (although you can fall through it from above if you use a door to glitch yourself through a wall in a burial chamber). Mistlands are mostly in a circle around the spawn area starting a touch over halfway to the edge of the world, primarily to the east and west of spawn, so get a boat and a portal kit and sail east or west till you find them. Advice- land near mistlands and walk in rather than sailing in on your initial explorations.