Server Assistant looks for machines that are booted into the initial setup screen. That won't do you any good. Server Admin is what you need.
If you open up a web browser on your server and enter localhost; do you see anything? If not, you need to turn on your webserver.
If you do, then go to another machine on your local network and type in the IP address of the server. Do you see anything? If not, you need to fix your firewall settings.
Go to a machine elsewhere on the internet and type in your external IP address. Do you see anything? If not, you either have your router configured incorrectly, your firewall configured incorrectly, or your ISP is blocking port 80.