The ayc filler plug is at the rear of the diff housing and high up. Its is obscured by the crossmembers, but is stamped ' OIL '.
Jack up the rear of the car, crawl under and look forwards to the diff, joggle your sight around over the crossmembers to the diff housing u should see it.
Removing it is easy, but replacing it takes a lot of patience because of limited access, but it can be done. Check your hands for fit BEFORE u remove it. You will need a 17mm ring spanner for the
filler plug and a 24mm socket for the drain. Remember the normal diff oil plugs are nearby , dont get them mixed up.
To fill it u need a large syringe and some tubing, or get some empty mastic tubes and make your own large syringe ( I believe RS Components now sell these ).
I have changed my AYC fluid on my e6 5 times now and have had no problems.
Good luck !