You have to finish the storyline quests in Wizard City. The last of these is to defeat Lord Nightshade. Once you've done this, Professor Ambrose will ask you to investigate Krokotopia. You might also want to stick around and do Colossus Boulevard, as the experience is good, and Prospector Zeke wants you to find a Smith on that street.
Yes, after you defeat Nightshade talk to Ambrose and he will give you the task called the sixth school (explore the death school in nightside). You get there by acting like you're going to unicorn way but when youre on rainbow bridge turn left and into the pond, then behind the waterfall. go through the door, move right a little and youll see the death school. once you talk to the guy inside and go back to the headmaster, hopefully he will give you acess to krok.