Currently, Free Tennis is only available as source code. You'll have to download it, then compile it.
(An ebuild for gentoo seems to be available here)
There are two ways to get the source code: the latest official release and the CVS repository. The version in CVS usually has more features, and it might have more, as well as fewer, bugs than the official source release. To discover which version is best for you, consult the two changelogs in the home page, which show the difference between the CVS version and the latest official release.
cvs -z3 -d:pserver:email@example.com:/cvsroot/freetennis checkout freetennis
This will create a directory called freetennis in your current directory, and put the source code in it.
The compilation was tested under Ubuntu GNU/Linux and Debian Sarge GNU/Linux, but it should be possible to compile under Windows, since portable libraries have been used.