Step 1: Start a game on MW3 Split Screen and run about for a bit and end it. Go to system settings, Memory, Configure USB to XBOX and then go to HDD, Games, MW3 and move the Saved Game to the USB.
Step 2. Open Horizon and click Device Explorer, USB, Games and then right click MPDATA and Extract to desktop. Minimize Horizon.
Step 3. Open MW3 Offline Editor and click File, Open and Inject your MPDATA which you extracted to your desktop into MW3 Offline Editor. Mod whatever you want. P.S: When you click Unlock All it will come up something like this " You will have to get a couple of kills with each gun to unlock all attachments, camos e.t.c "
You will have to do that, sorry. Click File Save and close MW3 Offline Editor.
Step 4. Open Horizon ( should be minimized ) and click once on the MPDATA and click at the top of Device Explorer " Inject New File " and Inject the MPDATA on your desktop over the one in Horizon. Finished.
Step 5. Inject your USB to your XBOX and sign into the account with the MPDATA on it. Go to Split Screen and Congratulations you are 10th 80. Remember to get a couple of kills for each gun.