The dipstick tube is there, that's where you add the fluid. It just has a cap instead of the traditional dipstick.

I've read anecdotal evidence that the cheaper ATF+3 fluid can be used in place of the more expensive dealer fluid. This is backed up by the fact that even though the Chrysler Crossfire and Mercedes SLK share the same drivetrain (including the transmission), the two different divisions and dealer networks recommend different fluids for the transmisssion.

Search for more detail on the dipstick. It costs around $40 from eBay. If you're resourceful, you can make one from cable wire for free. It works backwards from most conventional dipsticks. It bottoms out in the pan, and you measure the level from the bottom of the pan when the fluid is 80°C (not completely full hot).

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95 E320 Cabriolet, 169K