To add the EnviroDIY Mayfly board to your Arduino IDE, go to File > Preferences (for a Mac: Arduino > Preferences) and copy the following URL into the box labeled "Additional Boards Manager URLs":
https://raw.githubusercontent.com/EnviroDIY/Arduino_boards/master/package_EnviroDIY_index.json
Then in the IDE, click on Tools > Board > Boards Manager. Use the dropdown box for Type to select "Contributed". You should then see an option for "EnviroDIY ATmega Boards". Click the Install button to add the EnviroDIY boards to your IDE.
Now when you click Tools > Board you will see the EnviroDIY Mayfly 1284P listed at the bottom of your list of available boards.