Hey there, to edit with Assembly simply run it as admin and make sure it's updated to the latest version, if prompted by a window on startup to update then you should.
The campaign maps are the maps that start with the numbers and a letter, for example 01b_spacestation is Cairo Station, the map order is the same as the campaign order.
To make sure your changes take effect you can either hit "save" when not loaded into the map (in game) or hit "poke" if you are in the map, you'll need to restart the mission to see the changes take affect. Poking will not be saved after you quit the mission/close H2, so make sure to save your pokes once you're done playing the level - if you want those changes to be permanent for next time you play.