The pin is in a hole towards the top of the housing.
Remove the shroud around the steering column to expose the ignition housing. Towards the top of the housing on the barrel side facing the steering wheel is a small hole, big enough to fit a small pop rivet or similar rod. Behind this hole is a release pin which holds the barrel in place, stopping it from popping out.
In order to allow the pin to be released, the ignition key needs to be inserted and turned to position 1 - 2 depending on the barrel. With the key in position, insert an allen key into the hole and the barrel will pop out on its own (the angle of an allen key makes the job easier behind the steering wheel).