First download balenaEtcher (formerly known as Etcher), then follow the instructions in balenaEtcher to flash the moodlebox-3.5.1.img.gz file you've downloaded on your microSD card.
Copy with balenaEtcher
For the geeks
If you prefer to use a command line interface to flash your card: extract the moodlebox-3.5.1.img.gz file you've downloaded and follow the instructions to copy the disk image moodlebox-3.5.1.img to your microSD card.