The first adventure of Expedition to the Sky Citadel opens/appears on your campaign map once you've finished CardStock II (lvl 17 adventure). The rest of the adventure pack can be bought in-game with pizza.
Alternatively, next to where the GUI says how many pizza slices you have, press the red + button. It'll let you buy things directly, such as the Citadel Bundle which gives all its adventures, most of the character tokens/figures/costumes, immediate access to the high-level party that comes with the free adventure, three themed chests, and a month's Club subscription! Not a bad deal at all. I have all the adventures but these, and I'm only delayed here because of some .. unpleasant surprises last year and this year, but hopefully soon enough. $:^ J
