It order to do this, you must have version 2.0 of the
game. Start the game with the /edit command line parameter.
Go to a single player game and press [Ctrl] + E.
You will automatically go into the editor. Go to multi-player
mode and start a session. Join a map, then once in the level,
exit it. Then, click on "Chat/More". Note: This option
will not be there unless you are playing v2.0 or later. You will
be in the server again. Join a map again, and once in the level,
press [Ctrl] + E. The license agreement will appear.
You can now continue editing.