First of all, simply checkout the sources from the SVN repository.
svn co https://mavenizer.svn.sourceforge.net/svnroot/mavenizer/main/trunk/ mavenizer
To build the whole project and install all mavenizer artifacts in your local maven repository, cd to the root of the project and type
mvn install
Then to build the command line self executable jar, cd to cli/ and type
mvn assembly:assembly
The jar will be generated in cli/target/mavenizer-cli-$version.jar