I own Sonar Platinum and it's my understanding that the  PC4K-S Compressor module is included, yet I can't find it under my Pro Channel inserts.  Can anyone point me in the right direction?  
 
Thanks.

If I'm correct it's not included:
https://www.cakewalk.com/Documentation?product=SONAR&language=3&help=Plug-ins.1.html
 
I bought PC4K few years ago but don't use it anymore, as I prefer to use compressor like in the VC-64 and Nomad that come with Sonar.
Problem with PC4K is (in my opinion) you cannot set attack/release times properly...the increments are too big and you cannot enter values from keyboard. 

I found it in my Pro Channel insert drop down list but was propmpted to re-enter the reg code, accessed via 'My Account' from my purchase history. CCC didn't install it for me by default.  Hadn't noticed this till your post.  Working for me now, though.

I installed only the Platinum core on top of my previous versions of SONAR (which was the recommended approach at the time).  The PC4K-S module shows up in my installation just as it always has.  I bought that module back in 2011 according to my store account.  In my case it seems to be there and working only because it was previously purchased and installed when I upgraded to SPLAT.  No re-registration was required as it was in one of the above posts.  This information may not be of much help, but I hope that you can get this sorted out.

Thanks to all for the replies.  I've never purchased this plug in, but I had thought for some reason it was included in Platinum, your replies here make it clear that the only ones who seem to see it are those previous owners.  It's not worth $49 to me, I'll try to find a similar compressor.  I wanted to use this because it was a channel compressor with side chaining.
 
Thanks again to all.

I agree with your call on that.  I don't use the module much, if at all any more.  There are plenty of other capable compressors in the Pro Channel (or VST plugs if you'd rather use them).  

The PC4K S-Type Bus compressor is included in Sonar Platinum/Producer.  The PC4K S-Type Channel compressor is not.
 
The bus compressor is great, and gets used in most of my projects, particularly on master bus (as "glue"), drum bus, and sometimes on a snare track.
 
I use the channel comp less often, but sometimes find it is the only one to work in certain situations.  Sorry not to be more specific.  It took me longer to get used to dialling in this compressor, but once I listened, rather than looked, I learned to love it, and use it far more.  I have used it on vocals, clean electric guitars and acoustic guitars, to great effect.  I am pleased that I purchased it.

Thanks for the feedback, jb101.  I am looking to use the compressor on vocals and acoustic guitars so I might re-consider putting out the money.
 
Great feedback here and I appreciate it.

I got this in a bundle for 50$ with the concrete limiter a while back. Fantastic value, I'll say. Both are very usable, no nonsense plugins.

The Sonitus has side chaining. It's under DX fx.
For vocals and guitars I can also really recommend the Waves Renaissance bundle which goes on sale pretty often. The RVox, RAxx (guitars) and RComp are all great.