Summarize the following:
When you're connecting to the server from other computers on the same network, you'll need to know the server's IP address.  On the Mac running the server, click the Apple menu, select "System Preferences," then select "Network." Select your network connection and look for the "IP Address" entry. Make note of this address. You don't need to forward any ports or change other advanced settings if the other computer is on the same local network as the server computer. If your server computer is powerful enough, you can run Minecraft on it at the same time, but this isn't recommended for most computers. If you want to have your friends join your server over the internet, see the next section. This will begin scanning for available games. There's a good chance that you won't see your server available, even though it is. This will open a window allowing you to enter an address. After entering the address, you will connect directly to it and the game will load. If you cannot connect, make sure both computers are on the same network.  Multiple computers can connect to the same server using this address, as long as they are all on the same local network. If you are playing on the same computer as the server, enter localhost instead of the server IP address.
Determine the server's local IP address. Open Minecraft on another computer on your network. Select "Multiplayer" on the second computer. Click the "Direct Connect" button. Type in the server's local IP address.