I used to have Absynth available from the Synth Rack. I haven't used it for over 6 months, and now it is no longer present. Projects with Absynth on a track give the "Missing Plugins" error message.
Absynth is installed, activated, and runs standalone, but is not available in Sonar Platinum.
 
I have tried to re-load Absynth in the synth rack, but it is not seen anywhere.
 
Following the NI article "Inserting a NI Instrument Plug-In", clicking the "Insert Synth" button in the Synth rack results in a sub-menu with two choices: Insert Synth, and Insert Re-wire Device.
Insert Re-wire Device only has Melodyne SingleTrack, so I ignore it.
From the Insert Synth menu, another sub-menu is displayed. It has Plug-in Layouts, a divider, and a number of sub-catetgories from bass to synth. I have explored all those options, and Absynth does not appear anywhere. NI does not appear anywhere. vst64 does not appear anywhere. The NI article (I don't know how old it is) shows a sub-menu with choices "NI" and "vst64". Those do not exist in my Sonar.
 
From the Cakewalk Plugin Manager, I have explicitly added the path to "c:\program files\native instruments\vstplugins 64 bit".
I can see Absynth in the list of VST Instruments. I had to add it to a custom plugin layout, but now Absynth 5 is seen in the Plugin Menu Layout under Synth. 
However, when I try to add Absynth to the Synth Rack, Absynth is not listed on the menu under Synth.
How do I add Absynth to the Synth Rack?
 

Take a look here. 
 
https://www.cakewalk.com/Documentation?product=SONAR X3&language=3&help=Browser.10.html
 
Not sure if that link will work by clicking, you may have to copy and paste the whole line.
 
Have you tried clicking letter E "Synth Settings Menu" and changing your layout view to the custom one you came up with?

Thanks for the link. I noticed it was for Sonar X3.  I went up a level to https://www.cakewalk.com/Documentation/Home  and notice that X3 is the latest version shown.  Is there a different place entirely for Sonar Platinum?
 
I reviewed the local help (assuming it is for SOnar Platinum) for "Using the Synth Rack" and it is similar. The documentation seems to assume that the synth you want to insert is visible on the menus. 

Sorry, I'm not clear on your response. . . did you try switching the layout view to your custom one by using the little drop down menu in the synth tab?  And it still doesn't show Absynth available? 
 
If you go to the plugin browser synth section, can you see Absynth there?
 
Not sure why the default view in the Synth Rack isn't showing Absynth.   The drop down arrow should allow you to manage your layouts and you might be able to see more info in there.
 
I don't think there's a separate help section for Sonar Platinum.  It's kind of a mix of Sonar X2, X3, and Platinum.  Most of the stuff is still largely applicable though.

I just updated to 2016.12.  Now Absynth is back again!
May have been a bug in 2016.11.   I guess that's the downside of the fast update cycle. It's nice to get new stuff sooner, but possibly there isn't as much time dedicated to regression testing.