Go to Darkwhisper Gorge in Winterspring. And to the gate to Mount Hyjal. Now turn left and follow the cliffs to a pillar. On the right side of the pillar you can jump up on the pillars edge and walk around it. Then jump with mount over to a litlle area behind the pillar. Now simply climb from tere up on the roof-acting rocks and run over to Mount Hyjal.
Blizzard have put up an invisible wall preventing players to entrance there, but... Mage can easily blink through this "wall".
Do not despair fellow non-mages. There is a nother way
Standing on the hill between Darkwhisper Gorge and Hyjal facing Hyjal, climb right and get down on that side, that way you can pass the invisible wall
Now explore Mount Hyjal, with the craters, cool ruins and the World Tree (!).
Also take a swim in the lake underneath the World Tree, it's bottomless
Enjoy !