If it's an automatic you add fluid through the dip stick tube(with a small funnel).Also check the fluid when the engine is at normal running temp,while the engine is idling.Before you check it shift the car thru all the gears slowly,then wait two minutes before checking.Wipe the dipstick off then reinsert it and pull it back out,that will give you an accurate reading.
If the fluid is brownish and smell burnt you should have it changed.
If it's a manual,you add fluid into the left side of the trans.A squirt bottle works good for this.
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philvo