The unofficial Duolingo application will only run if it's in your Applications folder.
If you're not a Mac user the source code is included so you can use WebDGap to convert Duolingo for your OS of choice! However if you plan on integrating audio support please read this post.
FAQ:
This unofficial build of Duolingo for Mac was created using FluidApp which means that You MUST be using a 64bit Mac running OS X 10.7 or later.