Just a thought: Sometimes it takes hours or even days for domain registrars to propagate changes through the Internet. Your domain name may not resolve if it has been activated recently or if your dynamic DNS provider was configured very recently. If that is the case, it may just be a matter of time. You could try pointing your web browser to your public IP, on port 80 (XXX.XXX.XXX.XXX:80) then see what you get.
If none of the former is the case, try pinging your domain from the outside. Does it resolve? Do you get a response? If it doesn't resolve, try pinging your public IP. If you get pings back from your host, then there is probably something misconfigured in your Apache settings.