You may need to drop the barrier before you can use that teleporter (ie it may go somewhere behind the barrier, so doesn't work at this point).
Yggdragsil is the tree. You may not be able to talk to him until you get the quest to retrieve some seeds (in the cave behind the waterfall).
If you look at the cliffs behind the anti-dragon defenses, there is a bunker there you need to get to. One of the teleporters goes there, or it is possible to fly above the anti-dragon shield and drop down somewhere there in human form.
