You need to do this if you need the freshest version of Mixxx. However, it might have bugs. Consider installing the stable version from the repository.
First, clone the repo:
git clone https://github.com/mixxxdj/mixxx.git
cd mixxx
Next, install all of the dependencies detailed at:
https://www.mixxx.org/wiki/doku.php/compiling_on_linux
...you can compile it and do a system wide install by:
sudo scons install
Now you can run mixxx
by issuing:
mixxx