Should IIRC be two holes on them one fill and one drain, oh and a speedometer cable hole.
It is easier if you jack the car up and have a look. Approach the gearbox from the passenger side and there will be one hex bolt above another. These are the fill/drain holes and easiest way to fill it is to take the speedo cable out and the fill hole bolt, start pouring in where the speedo cable went and wait till it starts coming out of the fill hole.