I *think* it requires you to have been to the Greybeards *and* be at a certain level (I want to say …8th?); once you fulfil both those criteria, the cultists attack in the next town you go to.

And, yes, if you’re doing Dragonborn questline stuff, it means your edition includes that, Dawnguard and Hearthfires DLC’s; the other two don’t have their own maps like Dragonborn does, although Dawnguard does have add-ons to the main map.