Here's a screen shot.
Bare with me here, I know it looks half-assed right now. I'm saving the entire layout and graphics work for last. Most of the main features are easily distinguishable though. Starting from the left side, you have a list of blocks/objects that you can add to the stage. Under it is a preview picture of what you have selected. Below the picture is space for future options that will be associated with specific objects (ei: length). On the right hand side, there is a height selector. You simply drag the yellow box up or down to select the height to work in. Only the objects on that height level will be displayed. The objects directly below (and maybe above as well?) your current working height will be faded to help in building multistory structures. The stage itself behaves exactly as the in-game editor. You have infinite building space. You can zoom in, zoom out, rotate anything, etc...
The whole editor is made in flash. So to transfer what you made into the game you have to copy and past the code it spits out into the mission.sqm file.
Also note that this editor is not limited to only the features stated above. Those are just a few main ones I have completed; the finished version will feature a lot more functionality(like auto flooring).