Short answer - Pop/Rock Strings is a library - you will need Kontakt to use it! It is not a stand-alone product, with or without the Easy String Arranger add-on.
This might be a little dated, but I did check the KH web site, and it appears to still be the case...
Pop/Rock strings is an NI Licensed library - it will work in the free Kontakt player, and if you use it in the full version it will show up in the library tab. I can confirm this from personal experience. My version is still back at TVEC3.
Concert Strings 2, Concert Brass 2, and Diamond all require the full version of Kontakt, and none of them will work in the free player beyond the 15 minute demo timer, and none of them show up in the library tab. This is also confirmed from personal experience, and again all mine are still back at TVEC3.
That said...
I really like the KH CS2, CB2, and Pop/Rock Strings libraries. I like Diamond, and use it for woodwinds, but they are behind in terms of features and completeness, I know Kirk is just one person, but I really wish he'd update the woodwinds to the same level as CS2 and CB2.
And that's about the only negative thing I can think of!
CS2, CB2, and Pop/Rock Strings have a very distinctive sound, and they include all the major articulations. I was told that Pop/Rock strings is really a sub-set of Concert Strings, and I think it is probably true, they sound very similar. From memory there are some articulations that are unique to CS2 and others that are unique to Pop/Rock, but with updates that may have changed.
Please understand - these are NOT 'out-of-the-box' libraries. They sound good, but you do have to work with them to get them to sound great. I happen to think that's a benefit, some might consider it a problem.
One of the innovations I really like is the single patch approach used in CS2, CB2, and Pop/Rock Strings. You load a single patch and then select the division(s), articulation(s), and feature(s) you need by way of key switches, MIDI CCs, or even host automation. All three are available with Sonar and StudioOne, and likely other hosts.
Until recently I used key-switching, with an additional MIDI track mapped through a drum map, to help me manage key-switching. I am now on a kick to move everything to host automation. Why? First, keyswitches are limited to 128 values, and if you load the entire CS2 library, and want to control every feature, you need around 160 key switches. Oops.
While it is highly unlikely that a single project would ever use them all, I'm tired of creating 'custom' instruments for each project. With the advent of background loading in Kontakt5 it is now quite practical to just load all the instruments, and load the samples on the fly. (yes, it really works!)
Second - you can't reverse chase key switches, but you can reverse chase automation, host or MIDI.
Third - I still work with standard notation, and while the drum map approach works to keep the notation clean, it is a bit of a kludge.
The bad news is that you need an envelope for each change, I'm still trying to figure out a way to map all articulations, or all division sizes to a single controller message, but I haven't figured it out yet.
There are quite a few great sounding string libraries out there, LASS, Cinematic Strings, and Hollywood Strings jump to mind. All three of these sound better, out of the box, than CS2. But all three are more expensive! And really, all of these libraries have their strengths and weaknesses. If I was more successful as a composer I'd own most of them<G>!
All of these libraries represent one approach to libraries - the various instruments are available as individual patches. There is another approach, the antithesis if you will, where the patches are recordings of ensembles. I recently picked up Project Sam's Orchestral Essentials. It's really cool, and it sounds AWESOME without doing a thing. But you are limited by the instruments and articulations they provide.
Probably way more than you wanted to know...