The mods library downloads the maps into Document/My Games/Elemental folder whilst the map editor looks for maps in the /Maps where you installed the game, just copy and paste the downloaded ones there and you'll be able to load in the editor.

No idea about the save game maps, sorry.

As for save games, there does not appear (at the moment) to be any way to edit data related to a save game. Any xml changes only appear with the start of a new game. I"m hoping that gets some kind of "fix" to help in modding things.