The fluid level on the transmission dipstick is a pint low when it is at the add mark. You likely overfilled it. That would not cause it to go into gear though. If the car starts OK in park and neutral, then you can pretty certain that the linkages are set up correctly. With your foot on the brake, start the car and put it into reverse and let it stay that way for a minute or two. See if it slowly starts to want to move. The pump may whine during that time.