You use a taxi....

No really, you get the Quest from Soridormi in Caverns of Time, then you need to get 2 vials of eternity, one from Kael'thas in Tempest Keep: The Eye and the other one from Lady Vashj in Coilfang Reservoir: Serpentshrine Cavern. Good luck doing that =)

After you got her the vials, you're free to enter Hyjal.