Welcome to United Bimmer.
First, their is no dipstick because BMW considered them to be a "lifetime fill" They are NOT by any means. Their is a check plug on the bottom. With the car on a lift it is quite easy to access.
Castrol Dexron III meets the needs of your transmission. You probably have the A4S 310Z transmission, also known as the GM 4L30E, as it was made by GM in France. If this the transmission you have then Dexron III is sufficent. Check the bottom of the transmission pan for a sticker on the bottom denoting the correct fluid to use. Green is Dexron III.