NBTBRV8 - Rolf's interface consists of a USB connector at one end and 4 pins sticking out vertically at the other end. The USB is connected to your PC with an appropriate lead, and the 3-pin i-Bus connector going into your CD player (the most convenient place to access the i-Bus on the X5) is unplugged from the player and plugged into the IBUS/+12V/GND pins on the interface.

Jochen uses an alternative option of DIY 'jumper' cables to tap into the i-Bus lead with croc clips, with the other ends connected to the interface pins. Alternatively, you can do as some others have done by breaking into the i-Bus lead and creating a permanent and readily accessible flylead.