I suggest using the VirtualBox image as you mentioned, since it has all the settings ready configured.
If you want to make it work with a native system (note that unlike 1.8.3 firmware which only compiles on a specific version of Linux, 2.0.0 firmware can compile on all platforms), make sure you have added the fies in ‘arduino-code / hardware ‘ folder to your Arduino’s hardware directory. Also you need to select ‘OpenSprinkler2’ as the board in Arduino, as it contains the necessary mcu and parameters. If you select other boards, the code will not flash because Arduino will complain that it’s the wrong mcu.