See versions here:
http://www.bimmerboard.com/forums/posts/29257

You need v22 or later. Once you have the image, burn the CD with it using your favourite image burner

Note: BEFORE you insert the CD into the Mk3 nav, you MUST verify it is a Mode2 CD - use Nero InfoTool or any other CD tool to check the CD mode.
If you try and load the OS from a mode1 CD, the Mk3 will hang and won't boot.

Loading process:
Turn ign to pos1, let nav system boot up to display menu
Eject map CD
Insert new OS CD, follow on-screen instructions. DO NOT turn ign off during load process.