The tranny's oil pan should have a large, probably green, sticker informing of the correct oil. To fill: Put in lower plug (drain plug) and fill with BMW specified oil until it comes out the top hole (fill plug). The trans needs to be between 30 -50 degrees CELSIUS to fill to correct level. When trans is at that temperature, continue to fill oil until it once again overflows out the fill hole. Install plug and go. How do you know the trans temp? If you have a laser temp guage, aim it at the pan (duh). If no temp guage available... and you didn't hear this from me... run the car through all the gears, incl reverse, each for a few moments. Idle in drive maybe 4-6 minutes and then check level. It's pretty close. You should have a dealer or shop recheck it, but at least you can drive the car carefully.
What were you looking for under there that made you pull the wrong plug anyway? Just curious.