As djames said, hit Apple-Option-Esc, it will bring up the Force Quit (similar to End Task in Windows), and will show you the current applications running, select CoD2 from the list and Force Quit it. That should work for you.
Also, in regards to the Activity Monitor, what you can do is put a shortcut in the Dock so you can launch it easily at any time.
\Applications\Utilities is the directory of it. Just drag and drop the Activity Monitor icon to the dock for quick access. You can do that with pretty much anything in OS X.