Ok, I managed to create the link between maps, so that Map 1 is linked to map 2 etc through the proper triggers.
There's only 1 problem now, Map 1 is still marked as " Not Used " (Blue colour) and I cannot create a Map 0 (because then Map 0 would have that problem too) I think i gotta set a trigger to make the World Editor understand that Map 1 is the beginning of the custom campaign...
But what trigger? I can't find that.