Just look for the orange dipstick (arrow in picture). Do this on a level surface and with the engine off. Pull it out, wipe it, then stick it back in. Then pull it out again. The oil should be between the 2 line marks on the stick. If it's near the lower line (closest to the tip of the stick) or below, ad a quart (or liter depending on what country you're in).
To add oil, open the square cap just above and behind the "B" in BMW, near the firewall.