for the main menu u must edit “main.menu” 
When u only change the layot of the menu, u dont need the sourcecode.
U must only extract the pak0.pk3. These file is located in your Wolfenstein Enemy Territory/etmain Directory.
The pk3 files is a renamed zip file. U can extract it with RAR or my favorite: 7-zip…
when u extract the pk3, u will find then these directorys:
animations
botfiles
characters
fonts
gfx
and more…
U only need the directory names “ui”.
on these u will find then lot of XXX.menu files, menudef.h, menumacros.h, menus.txt and more
here a little ähm **** what is the english word for “Beschreibung?!”:
xxx.menu:
Here u can edit the menu.
The main menu when u start the game is:
main.menu
The ingame main menu is:
ingame_main.menu
menudef.h:
These file u dont need do edit it
Also forgott it 
menumacros.h:
These file u need, when u make new buttons.
Also i have make buttons without text and mirrors, but with a picture…
I am sure, u dont need to change these 
menus.txt:
When u add new menus, u must add the location of the new files 
Ah i forgott it: all XXX.menu, XXX.h and XXX.txt can u edit it with a normal editor.
My favorite is Notepad++:
http://notepad-plus-plus.org
So have fun in coding and modding on Et and good luck!
Greetz sCenna.