You are looking for a plug that either has a 12mm allen key head, or a 17mm bolt. Just look for a bolt about half way up the front of the gearbox that doesnt seem to be holding anything. And as stated fill it until oil starts coming out (also make sure you use the correct oil, you want a non EP oil).
It might be a good idea to drain whats in there first, the drain plug looks very similar to the fill plug, except its further down and is often on the back of the gearbox. Some of the newer gearboxes (my punto for example) have a 10mm allen key headed drain plug pointing backwards on the bottom of the gearbox.
A note of caution, the plugs are a taper fit, make sure you do them up tight (without stripping the thread in the gearbox housing).