What do you mean by codes? Offhand, I think you have the wrong impression of the engine and the editor.
Use the number keys on the number pad (make sure numlock is on) to modify cube geometry. They work differently depending on whether you're selecting sides of cubes, lines on those sides, or vertices. I believe that when you're in vertex mode you can also click on the vertices to drag them around. I suppose that could be useful for some geometric goals.
And if you don't have a numpad on your keyboard, go to the "view" menu and check 'editor toolbar'. It will pop up a pad of 10 buttons that correspond to the ten number keys on the numpad.
Remember that S selects side mode, L selects line mode, and P selects vertex mode (also O selects object mode but I don't think you need to worry about that yet
), and pressing S, L, or P again cycles between the sides, lines, or vertices of the current cube (hold shift while you press those keys to cycle backwards). Play around with it to try and figure out how all this stuff works. I don't have time to give detailed explanation.
I assume you've read the DLE-XP documentation on http://www.descent2.de
so you know the ins and outs of the more complex stuff. The basic documentation for DMB2/DLE-XP tends to be hard to find, but then we generally don't have new people come to the community with no knowledge of the editor, and yet wanting to do mapping for such an old game.