(adsbygoogle = window.adsbygoogle || []).push({}); First off you need Horizon.

Next you'll obviously need a multiplayer save, and then you'll need this level 50 survival level - You must login or register to view this content.

1) Go to the Xbox360 Dashboard
2) Then to System Settings -> Memory
3) Go to Modern Warfare 3
4) Move "Multiplayer progress for (Your Gamertag)" to your flash drive

Then...
1) Take the USB out of your 360, plug it into your computer then copy the multiplayer progress file to the desktop, and make a backup copy of it in case you stuff something up.
2) Open Horizon
3) Once Horizon is open you'll want to open the multiplayer data file of your own
4) Go to your desktop and drag the file into the root of your multiplayer data
5) Save, rehash and resign a few times then save it to your USB
6) Plug your usb into your 360 then go to survival and you should be at level 50.

I tried this, but when i put the download into my root horizon sent me a message saying "Invalid signature type detected for the loaded package. Halting reading. Please make sure the file is a valid Xbox 360 XContent Package ('LIVE'PIRS'.'CON'" HELP

yep thats why im never ganna hack MW3 im already banned from black ops first month