first of all where to download it?
http://viking.gurut.org/Map_Editors/, now that GPGNet is offline and you can't download it from there. Under normal circumstances, just downloading the exe should be enough.
where to install it
When installling Supreme Commander, it will create the following path in the location you selected: "...\Supreme Commander\Supreme Commander\bin" This is where it has to go. Be careful. When installing SupCom FA, you will get something like this: "...\Supreme Commander\Supreme Commander - Forged Alliance\bin". This is NOT where the editor should go. (If you put it there, it'll complain about missing files and even if you copy-paste those from "...\Supreme Commander\bin" to "...\Supreme Commander - Forged Alliance\bin", it'll not work)
then maybe links to some tutorials.
The best map editor Tutorial available:
http://supcom.wikia.com/wiki/Official_map_editor_tutorialIt contains all you need to make a basic playable map, add textures to make it look good and add decals to make it look better (although that is a painfully long process).
how to make a symetrical map?
Can be read in the 2nd lesson of the Tutorial
how is it possible to add such 3d things like on theta? how to add civilans
Can be read in the 7th lesson of the Tutorial.
how to make such surivel maps?
is it also possible to integrate a mod in a map?
These require some scripting. I haven't done it and it also isn't really explained in the map editor tutorial, but it's always a good idea to look at the .lua files of other maps. Take a survival map, look at the script.lua and I'm sure you'll find the text responsible for making the survival mission.
The problem with that is, is that you get a wall of text without an explanation as to what each line of code does.
The same thing goes for integrating mods into a map.
Finally: There has been a problem with the people who have downloaded supcom through steam. The map editor won't allow you to do anything (I think). The discussion was in this thread:
http://www.faforever.com/forums/viewtopic.php?f=2&t=2579And the file required to solve the problem can be found here (though I would like it if Viking could add it to his server, are you here Viking?):
http://www.mediafire.com/?360i8my8h6wzg5l