Just as you had to complete your required tasks in Wizard City before you could move on to Krokotopia, and just as you had to complete your required tasks in Krokotopia to move on to Marleybone, and just as you had to complete your required tasks in Marleybone to move on to MooShu, so must you complete your required tasks in MooShu before you will be given the key to Dragonspyre.
Gameplay in Wizard101 is story-based, meaning you must progress through the story in order to advance to new worlds.
I recommend being a lvl 40, Though you may get there at lvl 30 something. How to get there is most likely to do the main quest in MS to defeat the Jade Oni and talk to Ambrose and he gives you the key to DS.
I have a level 28 
wizard. What level do I have to be
Lvl 48