Matita has no officiale releases yet. Still, you can get and build Matita from sources. All our source code is released under the terms of the GNU General Public Licence and is publically accessible on our svn repository.
To checkout a copy of the sources type:
svn co -N svn://mowgli.cs.unibo.it/trunk/helm/software/
cd software
svn co svn://mowgli.cs.unibo.it/trunk/helm/software/components
svn co svn://mowgli.cs.unibo.it/trunk/helm/software/matita
Once you got the sources, have a look at the installation instructions on how to build and install Matita from sources.