Using Microsoft Windows and BlueSoleil
This site describes how to find your wiimote's bluetooth (MAC) address using a popular 3rd party bluetooth stack in Microsoft Windows called BlueSoleil by IVT. This section shows how to do it in the BlueSoleil's standard view. If you are using the Classic View, you should see this site.
Go to Bluetooth Places. Double-click Search Devices. Put your wiimote into discovery mode.
The wiimote is detected and for a couple of seconds, its MAC address is shown instead of the name. If you are quick enough, you can copy the address from here.
If you weren't quick enough, then right-click on the controller icon and select Properties.
The wiimote's MAC address is visible as Device Address.
Don't forget to remove the wiimote at the end so the computer won't interfere.