2003 C70 - Check trans fluid

[robotarmy]

I drive a 2003 C70. I want to check the transmission fluid, but I can't find the dipstick. Under the coolant hose on the right side of the engine is a yellow plastic 'button' that rocks back and forth (see illustration), but does not seem to want to pull out. Can anyone tell me for sure how to check it. The tranny has been shifting rough. About 117k on the well-maintained car.