Here is a picture of the ATF fill plug. It is right in the center of the photo and located almost directly below the brake fluid reservoir. It does say ATF right on it. I believe it is 17mm
The only reason I chose to fill through the atf dipstick is because it was already open and right there. I was hesitant without a ratchet extension to loosen that bolt way down in the engine compartment. Both ways work fine, just take your time and try not to spill the new fluid on your engine.
Here is another photo with lots of info that might help. Thanks nwf_snake