the best program for editing CFGs is: .........*drumroll*........... NOTEPAD!!!!

if you open a .cfg file in notepad you are still able to save it as .cfg
the only limitation is that you can't CREATE a .cfg file through it
but you can allways edit existing files
i see that you're trying to add a different livery to one of your planes, so here's how it's done
open the plane's folder, and paste your new livery's texture folder in it, but name it texture.myliveryname
open the plane's aircraft.cfg file using notepad, you'll see something like this:
[fltsim.0]
title=Boeing 737-700
sim=boeing737-700
model=
panel=
sound=
texture=
checklists=
kb_checklists=Boeing737-700_check
kb_reference=Boeing737-700_ref
atc_id_enable=1
atc_id=
atc_id_color=0xffffffff
atc_airline=
atc_flight_number=737
ui_manufacturer=Boeing
ui_type=737-700 - The 737 Experience
ui_variation=GOL PR-GOO
editable=0
description=The 737 Experience, with detailed virtual cockpit, Model by Moach. FD by Martyn Becker, Textures by Antony Totton and Moach
copy this entry and paste it right after it, above where it reads [general]
name your new entry [fltsim.1] (if' there is already one named fltsim.1, call yours fltsim.2)
in this new fltsim entry, change the texture= line to texture=myliveryname
chenge the ui_variation entry to the name of your new livery ex: ui_variation=mylivery
then save the file (notepad will keep the .cfg extension so don't worry)
open FS and select your plane, there will be a new variation with your livery
remember to always backup your original file in case something goes awry

hope it helps

c ya
Moach