If you have the 6 speed M20 transmission, level plug is right below the shaft speed sensor facing to the front. The plug is 10mm hex if i'm not wrong. There is also a filler plug (8mm hex) on top of the transmission, under the battery tray. You can fill it up there easily and verticaly if you're not lazy enough to remove the battery tray.
And like others have stated; you wanna change your GEAR oil, but NOT the HYDRAULIC oil.
The reason you wanna change the gear oil is because these M20 gearboxes are notorious with their bearing issues so keeping the gear oil clean is recommended for them.
For the hydraulics side; if you replace that oil, there is a chance that it can leak because the new clean oil will have a cleaning effect on the old seals. If that happens and the system loses pressure, you'll be stuck in Netural.
Change it when you are overhauling the dualogic unit and/or the transmission...