The washer fluid reservoir is behind the right side fender, so you'll need to remove the wheel and wheel well liner to access it. You can use a trim removal tool to remove the larger expanding rivets, but the three small ones on the outer edge are trickier, especially if you want to reuse them. The center portion needs to be pushed all the way through the rivet before the outer part can be removed, and if you do it gently, the center part won't fall too far inside and you can reuse them. For reference, this is part #10 in the diagram here:
2009 BMW 750Li Sedan Automatic Wheelarch trim
parts.bmwnorthwest.com
Once you get the liner off, you should be able to see where it's leaking from. This was as far as I got since my issue was evaporation, but there are more detailed steps in ISTA if you need to replace the reservoir or pump.