The aero plate should have been with your car when you picked it up. It is part of the car. The license plate bracket is separate and must be purchased separately ($15) from your dealer.
These are the retainers you use to install the aero plate (note the part number if you have to call to get them sent to you):
They should have been in the car with the aero plate and the instructions below (check the cubby hole in the back to see if it's in there):
You can use the aero plate to mount your license plate by bending the plate and drilling some holes. If your state does not allow you to modify the plate (i.e., bend it), then you need to use the license plate bracket that keeps it flat. It also raises it higher on the bumper for those states that require the plate located at a certain height.
Here's a picture of the aero plate
mrrrkva posted on another forum:
Here it is installed:
Here's the license plate bracket installed (notice how high it is):
