Welcome to the forums. The best thing to do to get answers is to post in the relevant secrtion. The general discussion has nothing to do with the flight simulator series. However, the top links on the main board will take you to the correct area for the Microsoft Flight Simulators.

Just post in the right subforum that relates to your game. Also include details of the operating system that your computer is running, and also which addon you're trying to install.

what operating system are you running?  you may need to turn off the UAC - User Account Controls in the system Control Panel.

when you try to add the 'texture entries' in the aircraft.cfg you will see information such as the following:

(excerpt from the default Cessna 172)

[fltsim.0]
title=Cessna Skyhawk 172SP Paint1
sim=Cessna172SP
model=
panel=
sound=
texture=1
kb_checklists=Cessna172SP_check
kb_reference=Cessna172SP_ref
atc_id=G-BAFM
ui_manufacturer="Cessna"
ui_type="C172SP Skyhawk"
ui_variation=" Blue, Gold"
ui_typerole="Single Engine Prop"
ui_createdby="Microsoft Corporation"
description= (blah blah blah - deleted to save space)

[fltsim.1]
title=Cessna Skyhawk 172SP Paint2
sim=Cessna172SP
model=
panel=
sound=
texture=2
kb_checklists=Cessna172SP_check
kb_reference=Cessna172SP_ref
atc_id=HB-MES
ui_manufacturer="Cessna"
ui_type="C172SP Skyhawk"
ui_variation=" White,Green"
ui_typerole="Single Engine Prop"
ui_createdby="Microsoft Corporation"
description=



Your NEW TEXTURE will have

[fltsim.x] <---- Change x to be the next number in sequence of [fltsim. ] numbers and put it along with the rest of the information below the last [fltsim. ] entry, Above [General].

once you have done this, click on the red X at the top, and it will ask if you want to save it. Yes, you want to save it.  It should automattically overwrite the file adding the new information.

If you are trying to write your own entries, and not copy/pasting them, that is a different story, and will take a little longer to explain.