I usually don't make guides but this site has helped me numerous amounts of time so this worked for me. Now I'm not saying my way is 100% correct but this worked perfectly for me.
Basically you need to install IOS224 to run MW3.

1. 1st download the precompiled link mod.zip by emailing simplisticmod@gmail.com (you should get a reply with links in a few seconds).
2. Go into the wad folder and grab the cios224 and put it in your wad folder on your SD card.
3. Install it using wad manager.
4. Download this http://mods.elotrolado.net/~hermes/w...ader_v5.1D.rar
5. Go in the apps folder and copy the cios_installer into your apps folder.
6. Run the cios 222 installer on your wii in the HBC and follow these steps.

To install IOS224
  1. Select custom IOS 224 v65535 to install, press the "A" button.
  2. Select IOS base, IOS 57, press the "A" button.
  3. Press the "1" button to install
  4. When completed, the Homebrew Channel will reboot.

Hopefully this helps because I know lots of you guys are dieing to play this game.