Instances in Burning Crusade are much more complicated than in ordinary World of Warcraft. There are a lot of steps You need to take in order to be able to get to Mount Hyjal. I started to read about it but it sounded so confusing. So I am happy that I was pointed at this thread on World of Raids Forum. Basically they made a chart with all the steps You need to complete to be able to get to Mount Hyjal.