Priests can't directly call crusades, the option is solely through the papal tab of the faction menu (where you can see diplomatic standing and faction ranking tables on the other tabs). If your relationship with the pope is good enough and it's been long enough since the last crusade then the 'call crusade' button will be active and offer a list of targets.
If you're playing as the papal states then (unsurprisingly) the papal tab won't work properly so there isn't a way to call crusades. Then again, you're also the pope with any catholic faction you war against being instantly excommunicated.