The Fairy-Max engine is available as Debian package and from the Ubuntu repositories. I don't know if your distribution can install such packages. If not, you could try to install from source. Which can be found in my on-line repository http://hgm.nubati.net/cgi-bin/gitweb.cgi
, from where you can load the latest 'snapshot' as tar ball. It contains a simple make file, so you can run "sudo make install" from the directory where you unpacked the tar ball right away.
Of course you could run XBoard even without Fairy-max installed by specifying an engine you do have installed (e.g. "xboard -fcp crafty" or "xboard -fcp fruit -fUCI"). Or using it in -ics or -ncp mode, which does not require an engine.