Just as you had to complete the required quests in Wizard City to move on to Krokotopia and you had to complete the required quests in Krokotopia to move on to Marleybone, so must you complete the required quests in Marleybone to move on to MooShu.
Simply complete the Marleybone world quests you have in your quest journal and you will progress as you should on to the next world.
