It's suposedly a sealed for life unit
but it has a filler pipe and combined drain level checker pipe, you run the gearbox and get it to 35/45C and undo the plug on the level check pipe, if fluid drips out it's ok, if it doesnt then you need to top it up via the filler pipe till it drips out the pipe
heres a link to a simplfied picture of the pipes
http://www.fordgalaxy.org.uk/ford/uploads/post-62-1128365966.png
the combined drain level check pipe is a two piece item, the first part you remove is nos 1 on the diagram, to drain the box you stick an allen key up the pipe and undo the pipe and all the fluid drains out!
to fill/top up you need to remove the snap lock cap and remove the cap/bung under neath it, again fill the fluid up till it drips out of the level pipe, then replace all the caps/bungs, run the gearbox till it's up to temp and then check the level, drain/top up as needed and bobs your uncle