You have to complete the relevant quest line for him to give you access. The quest line starts in the Palace of Fire, with the NPC right inside the entrance:
Gather the Troops -> Payback -> Give Em Another Round -> Back to Winthrop -> Into the Map Room -> Permission to Enter -> Fandango
You have to complete Permission to Enter in order to get permission to go to the Krokosphinx. If your level 18 class quest requires you to go to the Krokosphinx, that doesn't give you permission to go there until you've completed the relevant quest line.