If you need to find the MAC of an external adapter, it is often printed on the label. Otherwise 'All Programs' > Run > cmd > type 'ipconfig/all' > 'enter' and look for the right physical address as has been said.
My new Belkin G+ now works flawlessly on the WPA2 network - although it took a while to persuade the network administrator to fix his fault which was causing IP address errors! It hasn't been necessary to disable the internal wireless card whilst using the external adapter either.