You will need to symlink your addon into where ever you are hosting your base WHMCS installation.
ln -s /path/to/repository/whmcs-addon-template/src /path/to/whmcs/modules/addons/hello_world
You can access the client area default page at: http://your-url/index.php?m=hello_world
You can access the admin area default page at: http://your-url/admin/addonmodules.php?module=hello_world