If you have this wheel (or the optional 17" that is similar to this), you need the flat lug nuts.
OEM 16 Inch Rim
OEM 17 Inch Rim
If you have this wheel, you need the acorn lug nuts.
OEM 16" Steelie
Essentially the difference comes down to the mating surface. On the steel rim, it is a conical/tapered seat. So you need a lug nut that works with that too.
On the alloy rim, as the first picture, the mating surface of the lug nut to the rim is flat. Because of this, you need the flat (commonly called mag) lug nuts.