The original Mars Explorer 2.22 does not work on Mac OS 10.9 and newer, so here is an alternative version that works. It is the MS Windows application bottled with
Wine, and packaged to look & feel like the original Mars Explorer. It's pretty much fully functional, except that players may have problems connecting to games hosted with this app. It may take a little while to load the app.
346MB Disk Image; hosted in Google Drive.
Update: This does not work on macOS 10.15 Catalina or newer :(
Some alternative options: Use a Windows virtual machine; install Bootcamp; install Mac OS X <=10.8 on a separate volume if your Mac is old enough... also I've heard CrossOver and PlayOnLinux support Catalina now so you could try those.