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.
Each world will require you to complete certain tasks before you can advance to the next world. Grizzleheim you can visit at level 5, but cannot return there until level 20. Wysteria requires that you be level 25 before you can unlock that side world.
Your highest level Wizard is in the early teens, so it shouldn't be long now before you unlock Krokotopia. Keep working on the quest "The Dark of Nightshade" given to you by Merle Ambrose and it will lead you to the final quests in Wizard City.
Side note: if you were referring to switching REALMS, simply press Ctrl-M when you're not in a duel and select a Realm from the list. Each Realm will show a status (Full, Crowded, etc) so select a Realm that best suits how many players you want to be around.