Young Wizard, you must complete all your required assignments in Wizard City and Krokotopia before you will be asked to continue your adventures in Marleybone.
Start cleaning out your Quest Journal by complete the quests you have in there and you'll be in Marleybone in no time.
Trust me when I say don't be in a big hurry to get to Marley Bone, you will shortly after that be asking how do I get out of Marley Bone. LOL. Make sure you are at least a level 30 and it's even harder if you are a balance wizard since a lot of the enemies are balance as well. It's a hard world and A LOT to get through so rushing through it will make it even worse for you. I hope this helps.