You need to use the user and group that your local web server is running. www-data is an Ubuntu related one, maybe some other OS's as well, but its not OSX related.
Perms go like this
If memory serves me correctly on OSX you should do something like this
Don't forget the
Perms go like this
User:Group
If memory serves me correctly on OSX you should do something like this
sudo chown www:www file.name
Don't forget the
sudo chown -R www:www file.nameif you need to recursivley change.