The config is called 'customdvar6', IT NEEDS TO BE THAT NAME, DONT CHANGE IT!

first of all, place the customdvar in your mw3 folder, it could be;

C:\Program Files (x86)\Activision\Call of Duty- Modern Warfare 3 (this is the 4d1 version of mw3)
or
C:\Program Files (x86)\Steam\SteamApps\common\call of duty modern warfare 3 (the steam version of mw3)

How to open the config file in 4d1:

Open the winject folder (in the download) and open the program 'winject' (obv)
While opened, open your version of mw3. (tip: make it run windowed, in 1280720, u can easily change between programs, which is required)
once you have done that, go to winject. 
make sure you have these settings: http://gyazo.com/1b9c669d44456d59234919b7b1c7e665
the dll to inject is in the downloaded folder.
now click inject! a screen will popup now, thats the external console, no worries ;]
if u go in theater, pick a clip u want, and play it. switch to the external console and write:   !load6 
there u go, the cfg will show up!

How to open the config file in Steam mw3:

simply open your mw3, make it wondowed.. blahblah.. 
now go to the 'MW3 CONFIG\Barata's Console' folder and open baratas console (not the dll or 'settings')
a screen will popup, thats your external console. if u go in theater, pick a clip u want, and play it. switch to the external console and write:   !load6
there u go, the cfg will show up!

NOTE: if you want to remove the text/hud/chat for cinematics just copy+paste the config and rename it to: Customdvars7. now open the customdvars7 and write down (somewhere between the text): seta cg_draw2d 0
now u can use the customdvars7 for cinematics :)

THIS ALL COULD NOT HAPPEN WITHOUT BARATA!
thanks barata.. ;D