BALLS uses submodules, so clone it with:
git clone --recursive
When you do, run qmake from the root of the repository (or just open up the .pro in Qt Creator):
qmake ./BALLS/BALLS.pro
Finally, just run the resulting Makefile:
make
If the build is any more complex than that, please, please file an issue. Nothing pisses me off more than software that's a pain to install.
You can customize the build by adding flags on the command line like so:
qmake ./BALLS/BALLS.pro SOMEVAR+=somevalue
No such supported configurations exist right now, but they will.