1-c_str() converts a string into a constant char array(its what the compiler understands), yes you can simply replace GameWindow.c_str for "Call of duty 4" or anything else, make sure you are using the game's window name and not the process
2- I kindof showed this in the beginning of the tutorial, i believe this is the multi byte error. Go to "Project" tab at the top of Visual studio then click on "Properties" at the bottom, then below "Configuration properties" click General and look for "Character Set" within general and change from "Use Unicode character Set" to "Use multi-byte character set" and that should fix your error.
3- All i can assume is that you typed the variable name wrong this is the beginning of mine,
Make sure your "appWnd" is EXACTLY the same wherever you use it.
std::string GameWindow = "Counter-Strike Source";
HWND appWnd = FindWindow(0, GameWindow.c_str()); //rgb - Windows Photo Viewer // Call of duty 4 // Counter-Strike Source
appWnd = FindWindow(0, GameWindow.c_str());
std::cout << "Unable to find " << GameWindow.c_str() << std::endl;
Also if you do end up posting this aimbot on MPGH, I would greatly appreciate if you would give a link to the site where you learn this from, although thats up to you.