I think the idea is that you add it to the composer.json, and install it through composer. It doesn't work when you add it manually to apppath/vendor, the autoloader will not find classes in there.
I think it is already in there under suggestions...
If your composer setup is correct, you should have C:\wamp\www\alphacms\include\fuelphp\fuel\vendor\mustache\mustache\src\Mustache\Engine.php after installation of the mustache composer package. If not, your setup is incorrect.
I was not talking about the composer.json of mustache, but the composer.json of FuelPHP, which is in your installation root (there where oil is, and the readme...).