Click HERE and choose the latest release. You'll need the latest Java runtime to run the executible jar found in the release. For Non-Developers: after unzipping the release file, go to the Dist directory and double-click 'openflight_VER.jar'
For a bleeding edge version of the game, you can download the game's source code with a CVS client. You'll need ant to compile the code.
simply paste this into console:
cvs -d:pserver:email@example.com:/cvsroot/openflight login
(now press enter when prompted for a password)
cvs -z3 -d:pserver:firstname.lastname@example.org:/cvsroot/openflight co openflight
If you dont think you have java runtime 6, or if you are not sure, you can just MAKE sure by downloading it and installing it.