Hello and welcome to Bimmerpost!
The rectangular box is a sensor array for the active driving assistance features. It is optional equipment.
The US license plate will be located in the same place as the license plates are located in other parts of the world - on the crossbar in the grill.
Many words have been written about the grill in this forum, and you can find a wealth of information using Google searches like:
https://www.google.com/search?q=site...m+grill+sensor
https://www.google.com/search?q=site...om+grill+plate
Feel free to play with the search terms to narrow things down.
Enjoy.
