After patching (if necessary), the Launcher closes when 'Launch' is clicked but Descent: Underground does not launch.
On OS X machines which don't have the Simple DirectMedia Layer library (LibSDL) installed, the Game is unable to run. LibSDL is meant to be installed automatically but has been known not to install.
To remedy this situation on a Mac runninng OS X, you will need to install LibSDL. You will need to install the .framework bundle in either your /Library/frameworks or ~/Library/frameworks directory.
You can download the latest DMG file for Mac from the Simple DirectMedia Layer website.
Once LibSDL is installed, please attempt re-launching and you should be able to enjoy the game.