In Krocktopia, codes are usually on walls going towards enemies or in a room of an enemy before the main enemy. The walls will be a bright gold color. If there isn't a code, read the book to find out what one must do like changing the flames a certain color. If I have a student do this quest again, I will put more details.
Heading towards Sertek's area, there are tall panels with images on them. These same images are on the oblisks at the entrance to Sertek's tomb. Make sure what is in the hall and what is on the oblisk are lit.