Note also (since no one has mentioned it) you need to check the level with the engine running. Opening the fill port with the engine off will cause a large fluid loss and then your top up just got harder. Don't open the fill port until the engine is running and you've cycled through all the gear selector positions (P, R, N, D and back again).
And if your ZF 6HP26X is like mine (and I'm betting it is...) then the fill port is in the bottom of the pan. You'll need a fluid pump and a good idea of how to get fluid into the fill port.
