Hi Matt,
If you want the salinity probe, it is indeed an add-on. If you are willing to go that route, I would suggest looking at the Neptune Apex "Gold" package, it contains the standard APEX, with temperature, pH, ORP and salinity probes.
The salinity is just measured using a probe similar to any other measure, which gives a conductivity number. But this number is translated into salinity. Aka "ppt". (for the hardcore guys who want the conductivity number, you can still get it, it is a setting).
It is a somewhat delicate probe however, you need to make sure that it is fully submerged, and that no air has made it inside. But once set it is pretty easy to use (some do recommend to recalibrate it more often than other probes).
ps: on a side note, nobody has mentioned it here, but the GHL Profilux is also a very good controller. Which comes packed with plugs for every probe imaginable. However it is harder to find, and well... the translation from German in the manual can be "interesting".