On most automatic gearboxes you top up or refill via the dipstick tube - small bore tube and a handpump at the repair / service shop.
Also level is normally checked with (following putting most of the specified quantity in first) the engine running.
Check you handbook some gearboxes have filters that need to be replaced at specific mileages. This may mean dropping the gearbox sump replacing the filter and sump gasket.
Maurice
April 2011