The basics for internet sharing with your Mac to another Mac:
Go to the Internet Sharing tab in the Sharing Preference Pane.
1. Choose the connection to the internet (we'll assume wireless in this instance).
2. Choose another connection to share the connection (Assuming ethernet)
3. Press Start.
4. Open ports on your firewall (eg. port 80) so the other computer can access the internet.
To confirm it's working on the second computer, confirm it is getting an IP address that is not 169.254.x.x (where x can be any number less then 255).
Are you able to confirm this much? If the problem is after this it is most likely to do with your firewall.
You need to enable NAT on your settings within the server admin tab. The IP forwarding doesn't matter at this point, but you can add it in the hostconfig file which is located in /etc/
If your WAN interface is en1 (which is your airport) you might have to share change the PLIST configuration on the NAT settings and change the settings from en0 to either en1 or ppp0 if the server is connected directly to the WAN interface.
Obviously your DHCP would reflect this internal IP address that you want to share out across the Built-in Ethernet interface.
As for not getting your other machine online, is your dhcp service handing out IP addresses from it's pool? Can you pont the internal IP address (192.168.1.1) for example, and if so can you ping the WAN IP address that is provided to you by the ISP???