To add the app to your flipper you first need to download the code.
1.Press the green button above that says Code then press download zip
2.Once you have download the zip file you need to extract it. In the file you will find two folder one labeled esp8266_deauth and Flipper Zero App. The esp8266_deauther is the code for the esp8266 board. To download the flipper app open the folder named Flipper Zero App. In that folder you should see a .fap file. Thats the app.
3.Next go to https://flipperzero.one/update and download qFlipper
4.Once you have qFlipper downloaded open that up and plug in you flipper zero. Once in qFlipper go over to the file tab (The last tab at the top left). Once your there go to SD Card>apps>GPIO now drag the .fap file into that folder
Thats it now the app is download on your flipper and you can access it by going to Apps>GPIO>[ESP8266] Deauther