Make sure that the npm directory is in your "Path" variable.
If the module is installed properly, it may work if you start it out of your global node module directory, but your command line tool doesn't know where to find the ng
command when you are not in this directory.
For Win system variable add something like:
%USERPROFILE%\AppData\Roaming\npm
And if you use a unix-like terminal (emulator):
PATH=$PATH:[path_to_your_user_profile]/path-to-npm