The MOOS-IvP development repository is sadly still SVN, sitting on a computer somewhere in MIT.
The mirror branch is a mirror of the MIT SVN development repository, but filters out a lot of auxillary files including the MOOS snapshot.
In an attempt to keep the repository small and quick to download, this mirror is only the source code used to compile the libraries and applications.
If you need missions and the files associated with the course you should obtain those from the SVN repository directly.