Your Mac probably has 2 MACs.
One for the Ethernet adaptor and the other for the wireless unit. They are not the same.
When you attempt to logon to the access control enabled WIFI network, the access point or router would authenticate you VIA your wireless adaptor's MAC address and not the ethernet one. If the incorrect ID is used, the network would refuse your connection.
You may also discover the needed numbers in the settings area of the Airport application on the computer. Look for the Airport ID.
Good Luck!