Rene,
We do most of our development on a localhost configuration. You may run into some issues with Chrome (they can store localhost cookies strange), but other than that, you shouldn't have problems.
Since you're not using the standard port 80, you may need to play with some settings, but the warning above is just that.. a warning. Things may still function just fine even if we detect possible problems there.
Basically, on the front-end, if you get the normal Facebook login pop-up, you're configuration settings are correct. If not, play with your Site and Domain URLs in your Facebook application. Some combinations to try:
Site: http://localhost:81/
Domain URL: localhost:81
Site: http://localhost/
Domain URL: localhost
Site: http://localhost:81/petdietdesigner4.com/
Domain URL: localhost:81
Site: http://localhost/petdietdesigner4.com/
Domain URL: localhost
One of those groups will hopefully get you going. If not, let us know!
Thanks,
Alex