Assuming you actually installed the IDE correctly, open it up (you can search for "Arduino" if you can't find it by other means). You'll be presented with something like this:
Now, go to "Tools" -> "Board: Arduino/Genuino Uno" (if it actually says that) -> "Boards Manager..." as shown here.
So to "clara"-fy, this class uses the Arduino Due boards. These are outdated models and are not initially supported by the Arduino IDE.
The Boards Manager allows us to install additional modules, and one of them supports the Arduino Due. At the top where it says "Filter your search...", type "Due".
It should filter the results to the modules that support the Arduino Due. The "Arduino SAM Boards (32-bits ARM Cortex-M3)" one is what we want. Click it and an "Install" button will appear. I think it's obvious what you should do next...
Restart your Arduino IDE, have your Arduino Due plugged in, and be prepared. We are going to test if it actually works.