That tutorial is completely wrong. (And that's one of the sites for which this seems to be very frequently true.)
In EPEL, the unison package names include a version number, such as unison227
and unison240
. This is because, for whatever reason, the various versions of unison are not compatible with each other, so the version must be the same on all systems to be synchronized.
On EPEL 7, only unison240
is available, and at present it's in the EPEL testing repo and hasn't yet moved to stable. So that is the base name of the packages you will install (e.g. unison240
, unison240-text
and/or unison240-gtk
).
yum --enablerepo=epel --enablerepo=epel-testing install unison240