Plug your mac mini into the VGA port (Its in the middle of the top section, with the smaller flap), and just assign PC to AV5 or AV6. You will need to tell the input it is using the VGA video input. Don't forget you need to connect some phono leads into the relevant AV port in order to get sound.
You need to use LIST > PC on the Beo4 or add a PC button on your Beo5 if you have one to select the source. Its very easy to do - If you need anymore info just ask.
There is a PUC table for the Mac Mini to give you some control over it using the Beo4/5 but you would need a B&O engineer to come and squirt the software into the system 3 and install an IR cable in order for it to work.
Hope this helps,
Llewelyn 