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. Go there and download the tarball of the components/ on which Matita is built and of matita/ itself.
Once you got the sources, have a look at the installation instructions on how to build and install Matita from sources.