Without getting too technical, hole at the top mate - I know its easy to say but with the wheel off its hopefully kinda obvious. beware - the fastener is tight in (its a socket screw - M8 I think..........)
Think you will find the bit labelled 9 is the filler plug, it is located in front of the ABS sensor cum speedo sensor. You need to remove the wheel to find it.
I had a suspected problem with my final drive on a trip to the US. Portland BMW mistakenly told me that I would need to replace my final drive at some point before the end of the trip, and that I should keep an eye out for oil leaking. Sure enough, days later I spotted an apparent oil leak, panicked, and rang them for advice. We were 400 miles from the nearest dealer by that point. They told me I should be able to get to either Vancouver or Kelowna "as long as I kept the drive oil topped up". "Fine", I said, "how do I do that?" "You need a special syringe", he said, "as the filler cap is underneath the drive". Very helpful. (ETA - OK, looking at Magwych's post, this is clearly incorrect, but you get the gist.)
In fact, pleased to report that I didn't need a new final drive at all. The muck and crud that the Portland dealer had discovered during their "inspection" were down to ... wait for it ... a perished rubber boot!