1. Extract the crack with
WinRAR
2. Copy the crack over to the installed directory (Usually something like C:\Program Files\Call Of Duty. You will know that you are copying to the right folder because you will prompted with a Window telling you that a "file with that filename already exsists", do u want to replace? Click yes. Also, its a good idea to make a backup of any files you are going to replace.
3. Play the game!