Your level doesn't matter. Every world in the spiral is gained access through questing your main storyline quests. There is no other way. Finish Krok, then Marleybone and you will gain access to Mooshu. Don't be in such a hurry... enjoy yourself - this is a game, after all.
You don't have to be a certain level to go to Mooshu. But you do have to complete all of the Main Quests in Marleybone first. When you finish the last Main Quest in Marleybone, and you defeat Meowiarty in Big Ben, then you will be sent to Mooshu.
This link shows all of the quests in Marleybone, in case you need more details: