Moka is available on the SVN server of SourceForge. To download the sources, you only need to use the following command svn checkout svn://svn.code.sf.net/p/moka-modeller/code/ moka-modeller-code to get the last stable version of Moka.
If you do not have SVN, you can go to web site of Moka and download the tar.gz archive containing the last stable version of Moka.
When you use SVN, you can update your local version of Moka when modifications are available, only by using command svn up in the root directory of Moka.
For more information on SVN:
SVN is available on all the linux distributions (for example for Debian, you can install it by using apt-get install subversion). There is also several versions for Windows (for example TortoiseSVN).