Today I managed to upload the AHRS firmeware to the Razor 9DOF IMU. To achieve this you need to download the arduino platform software and of course the new firmeware (see link in previeous post). You also need to connect the 9dof IMU to the pc via a usb to serial uart (FTDI) converter.
The converter I use is this one: http://www2.conrad.be/goto.php?artikel=197326. This converter can operate in 3.3v or 5v TTL signal. The razor 9DOF IMU uses a 3.3v signal on the TX and RX pin. To power the IMU board you can connect the vbus pin from this converter to the external powerconnector on the razor imu board. So no need for an external power supply because all boards will be powered by the usb connection (max 100ma for this ftdi board).
More info about uploading the new firmeware to the 9DOF IMU can be found on this step by step tutorial: razor firmaware upload