MCX is an open-source software. It is released under the terms of GNU General Public License version 3 (GPLv3). That means not only everyone can download and use MCX for any purposes, but also you can modify the code and share the improved software with others (as long as the derived work is also licensed under GPLv3 license).
If you have made a change to the code to tackle a bug you have encountered in your research, we are appreciated if you can share your changes (as "svn diff" outputs) with the developers. We will patch the code as soon as we fully test the changes (we will acknowledge your contribution in the MCX documentation). If you want to become a developer, please send an email to Qianqian Fang and we will review your request. Once permitted, we will grant you developer access to the SVN repository.