The fill plug is on top of the transmission closer to the firewall. It's a large Hex bolt. If you fill it, make sure that you replace the sealing washer on it. I believe the hex is 22mm.
Just so you know, you have to check the level with the car not running, unlike most other vehicles. Also, if it's low, you should look into where it's going. For fluid to leave, there has to be a leak somewhere.