DuneRL is a free and open-source survival "roguelike" game with large areas of sand dunes for you to explore.
The aim of the game is simple: you need to find your way back to your base, after being lost in the desert. You have a limited supply of wate and you can collect and consume spice to help with the dehydration.
Detailed instructions on how to play DuneRL on your Mac are available HERE.
DuneRL is cross-platform and it works on Mac OS X and Windows. Binaries for the Windows platform are available on the project's homepage.