Yes, you've got the right coolant tank, its right at the back alongside the windshield wipers for the 540i. Quite a dumb place to put it, but i guess they ran out of space near the front. Is prestone sweet smelling and brightly coloured? If so, use that, if not, I suggest that you get a brightly coloured sweet smelling coolant and ad it to your tank for reasons i've detailed in my thread linked earlier.
Anyway, why are you thinking of adding coolant? Are you running low there?
In any case, make sure you open the tank cap slowly if the engine is hot. Opening it slowly will allow the internal gas pressure to be release gradually, preventing hot coolant from shooting up when you finally remove the cap.
In any case, for greatest effectiveness, add coolant to the reservoir when the engine is cold.
The expansion tank you see should have a hot and cold level mark. As long as your coolant levels stick between those, you're ok.