The creatiion of campaigns with StarEdit 2(Galaxy), is done via map specific triggers.
Basically...
Make one map as a starting point...
completing the map sets the "Set next map" trigger.
or have two possible ways to complete and two/three possible "next maps" depending on how you played the map.
From what it seems...
Blizzard made their campaign by using a single map called "Storymode" that would ALWAYS be the next map, and it would load a bank at the start of the map each time you returned to it to load saved information on which maps had been recently completed and variables that control what part of the story you are on, and what paths you have chosen.
Unfortunately this limits map makers to 1 campaign launching map, and 4 actual maps, due to the 5 map limit in place on the publishing system, UNLESS you work with many users who all uplaod a different part of the campaign.