STEP 1: Go to Multiplayer Menu, go to Xbox Live and chosse Theater
STEP 2: Go to Switch Lobbies and hover over Private Match, but dont press it, just press (B)
STEP 3: Go to virtus wezzlan's Cod Playercard and select the file that you wanna play in Combat Training. (Its in the Sixth Slot)
STEP 4: Go to Fileshare and Load the file
STEP 5: Once you Load the file it will take to Private Match and start the game.
STEP 6: As soon as the game begins end the game immediately.
STEP 7: Press Up and and click Fileshare quickly and Load the file again.
STEP 8: Switch Lobbies to Combat Training.
STEP 9: Then Switch to Theater
STEP 10: Go to Private Match and and Load your file again.
STEP 11: Switch to Combat Training.
STEP 12: Choose Switch Lobbies and Hover over Private Match, but dont press it.
STEP 13: Press (B)
STEP 14: Go to your Playercard.
STEP 15: Once your Playercard is up, go to Fileshare and load the game, but before you Load the game keep pressing the (A) button rapidly.

*All credit goes to IDutchy, I am not taking credit for this find and I am just trying to clarify the misunderstandings with the instructions and I hope this clarified it. Thanks and hope this helped:)