Once you have completed all your required assignments in Wizard City, Merle Ambrose will ask you to investigate Krokotopia.
Once you have been given that quest, you can access the Spiral door by entering Bartleby the World Tree.
Much like reading a chapter book, Wizard101 is sequential game play (before you move to level 2 you must complete level 1).
Even if you have purchased a Membership you must first complete all of the quests in one world to be able to move on to the next. The same thing goes for Access to some of the streets or areas within a world. Once you have earned your own access there, as long as you have a Membership or have purchased the Premium Zone with Crowns, you will be able to enter that world.
But with a membership if you have a friend in an area you haven't seen you can port to them earlier than you are able to go there. But you just can't get there on your own.
--Jack Frost, Imp Realm--