To open the editor you must open the console by pressing CTRL + ALT + ¬. Then type the word EDITOR and press enter.

You will find that it is zoomed in and you wont be able to use. Now press F2 to return to game, go to options, system, and switch the fullscreen off.

Now quit the game, open it again, and open the editor again. Now yoiu should be able to see it proparly.