There are many ways to get access to the source code. As a 3rd party developper (user) you can download libmatroska and libebml from there official location in .tar.bz2 format. If you are a Java developer you can checkout JEBML from our Git repository for a Java Matroska reading library.
You can clone a Git repository with the following command:
git clone https://github.com/Matroska-Org/repositoryname.git
e.g. for libMatroska:
git clone https://github.com/Matroska-Org/libmatroska.git
Some of the very old repositories have not been migrated to github.com. The source code for those is still available in our old Subversion repository which you can clone with the following command:
svn checkout https://matroska.svn.sourceforge.net/svnroot/matroska/trunk/