It won't be in the network preferences for your own computer or any other computer. MAC address filtering is done at the network router. Go into your router administration settings and look for a MAC address filter section.
If MAC (Media Access Control) Address Filtering was turned on by somebody for security reasons, turn it off. While you can use MAC address filtering to keep out unwanted users, it isn't very effective because it's very easy to get around it. If you already secure your wireless network with WPA2, it's already a lot more secure than it would be with MAC address filtering anyway.