Take care of the mod_unread module configuration on you ejabberd config, otherwise the module won't be started on your instance and you cannot use the features. The second important note is the database migration. Execute the SQL statement on your database which was bundled on the binary relase package.
The ejabberd project is able to compile and load contribution modules at
runtime. You just need to download the source of this module into
~/.ejabberd-modules directory or the one defined by the
CONTRIB_MODULES_PATH setting in ejabberdctl.cfg. There you create a
directory named mod_unread.
Then run ejabberdctl module_install mod_unread while the ejabberd
server is running and you should see a logged info about the unread
module was started. Then you are able to use it.
Just run ejabberdctl module_uninstall mod_unread while the ejabberd
server is running and delete the mod_unread directory from your
contribution modules directory (~/.ejabberd-modules).
Be sure that the ejabberd package is installed correctly via apt. Then you
can use the following curl-pipe command to automatically install the module to
the ejabberd server. The server MUST be restarted afterwards.