Save the file to desktop, edit it there, then drag it back to the folder.
Alternatively, give yourself permission to edit the folder:
you have to go to where rome is located (probably C - program files) and then right click on the The Creative Assembly folder, select properties and then there should be something like security and in that menu you can edit the profiles (user, system etc.). So you can give yourself permittance to edit the folder (and that means everything inside it).
Or download a pre-edited file and replace it:
http://rtw.heavengames.com/downloads/showfile.php?fileid=4450