What do you do after you had crossed the river styx and had gotten Cerberus' whisker in poptropica?

If you have all five items go back to the place you first met Athena, and talk to her.

Her father, Zues will appear and take all of the items after you figure out the code thing that Athena has you do. Then get heraclese to help you he will open Hades and Posiedons palaces Zueses also then you have to beat Zues and your done!