It is quite hard to see but it is on top of the diff towards the back. You can see it from above just about with the plastic engine cover removed. It was easier to get to from underneath when on a ramp by reaching up over the gearbox and feeling around for it.
I filled it up by fitting a small fuel pipe (same stuff I use for bleeding brakes) to the tube on the gear oil bottle and feeding the other end into the dipstick hole then hanging the bottle like a drip from one of the posts on the ramp and cutting the bottom off the bottle
I've seen a V6 gearbox filled with something like this, but I'm not sure if ths particular one has a long enough hose
That what I used, Bought from wilko for around £1.20 each
Used the snorkel of one and the whole of the other on top.
Whilst your at wilkos, buy a pastry brush for 50p, great for cleaning door sills etc.