Right-click the application file and click Open on the context menu. From then on you can open it normally. (On newer versions of macOS you may need to allow it to open in the Security section of System Settings.)
Why doesn't the game open (Linux)?
You may need to make the Tinybox.x86_64 file executable. On some Linux desktops, you can right-click the binary, access Properties, and check an 'executable' flag. Otherwise, open a terminal in the game's folder and run:
chmod +x Tinybox.x86_64
to make it executable.
How do worlds get featured?
High-quality, unique, or inventive worlds may get featured by an admin. These will show up at the top of the map list and on the website.