The gearbox oil should be checked for level occasionally, especially if you have a leak. Otherwise it should be changed evey 105,000 miles or 7 years.
I have just filled mine up as I have just fitted a new gearbox and drained it to fit a magnetic drain plug.
The oil I used was Castrol SMX-S.
Filling from empty takes around the 2.4 litre mark, so you will need 3 x 1 litre bottles.
The bottles usually have a filler tube built in. In this case you simply break the seal and pull the centre cap outwards.
The filler hole is also where you check for the level. It is above the drain plug and to the right, looking from the nearside.
You can see them on this pic of my old gearbox with the driveshaft removed.
If you are checking the level, then it should be just below the threaded hole with the filler plug removed.
Unfortunately, in real life, the filler is a pain in the but to get to.
Remove the filler plug from underneath the engine bay (17mm socket). You will need the car up on axle stands or ramps to do this, but the car must be level when checking or filling the oil. If any oil comes out then the level is OK (should only be a slight weep).
I found the easiest way to fill the gearbox was to remove the engine inspection cover and the airbox. I also used a length of tube, one end pushed into the filler
You can also see the drain plug on the above photo, it is the shiny one as it is an aftermarket magnetic plug fitted on mine. If you are changing the oil then you will need to remove this and drain the oil into a suitable container, refitting the plug afterwards.
The other end of the tube is accessable through the top of the engine bay. Simply fill the geabox from here.
Now, just keep filling the gearbox until the oil starts flowing out of the filler hole.
Once the oil has stopped dribbling out, refit the filler plug and the gearbox is done.
Now refit the airbox and engine cover.