If you have a ULF (Bluetooth) > 2nd GEN (2004) installed and VOICE activation is not working, you can use Navcoder (as previously suggested) OR visit a dealer and ask them to activate it, it's a 15 minutes job.
You can check for your ULF version in the LEFT side of the trunk.

Details on ULF versions can be found http://www.bimmernav.com/ulfchanges.html