Look in the service manual starting on page 21-307 for the description of the transmission dipstick cap & lock. You can buy a dipstick($$$$) or use a heavy duty zip tie like Woody documented.
My car is not here right now so I cant go take a pic of the drain plug. It should just be on the bottom of the pan and easy to find.(assuming you know which part is the transmission) Be sure to replace the drain plug seal when you re-install it. The service manual gives fill levels based on temp of the transmission fluid.