Check the auto trans pan sticker for correct oil used, if a bmw part number is quoted please see you nearest bmw dealer, if automatic transmission fluid/dexron ATF is quoted your average ATF will do the trick.
From memory you should be able to add trans oil through the auto trans dipstick tube.
To check the oil, reach normal operating temp, run through the gears stationary and leave in neutral idling and check via dipstick
These cars have a sealed transmition and you need a specific Trans fluid. It's extrememly expensive for fluid. I paid $157 at the local dealership. I looked through every other option to get a hold of it but it was only available throught the dealer.
Fluid part # LA2634
This is for my 93 BMW 740i RWD Automatic transmition.
As far as how to add fluid, I have yet to try doing it but there is a process. If you search around more you will be able to find what the process is. I don't remember off hand and I don't want to tell you the wrong information. I can tell you this, It ain't easy.