Ok. Here goes:

Right click on your RTW shortcut on the desktop.
Choose properties
In the "target" section add the -enable_editor switch so that the whole thing looks like this:
"C:\Program Files\Activision\Rome - Total War\RomeTW.exe" -enable_editor
NOTE: do not put "" around the switch. Just leave one space and type it in. This should do it. Once you start the game the battle editor can be accessed from under the options menu.

Further switches can be added on the end as well, thus:
"C:\Program Files\Activision\Rome - Total War\RomeTW.exe" -enable_editor -ne -show_err

PS, CTRL-Z and CTRL-Y are the most useful commands. Undo and redo!