I had spent endless hours trying to resolve the issue of SessionDrummer not appearing in X3 studio. None of the advice I found here and elsewhere helped.
I then noticed in Sonar X3 > Edit > Preferences > File > VST Settings a checkbox captioned "Hide Related VST2 Plugins". The box seems checked by default. 
 
I unchecked it and SessionDrummer appeared!?
 
I should emphasise that this is on a clean install of X3, i.e. no Cakewalk software had previously been installed on this Win8.1 OS.
 
My experience would therefore suggest that SessionDrummer is identified as a VST2 plugin. 
 
It would have been nice if this was noticeably stated somewhere. It would certainly have saved a few hours I'm never going to get back.
 
Anyway, I hope this helps someone.
 
 
 
 

maybe this will help http://www.cakewalk.com/S...recognized-by-SONAR-X3

Thanks, but as I wrote, I had exhausted all that and more. My post was to highlight the fix that worked for me and is not mentioned anywhere else AFAIK.
 

I suspect it is not mentioned because the setting you changed is a VST3 migration setting. The VST3 migration settings affect how Cakewalk handles VST2 plug-ins when a VST3 plug-in replacement is available. Session Drummer is VST2 only.

How can it be a "migration" when it is a clean install?

Because the migration settings have nothing to do with installation but how SONAR migrates projects containing VST2 plug-ins to their VST3 counterparts.

?? Now I am totally confused. 
 
Anyway, it's working.

Here is a blog explaining the settings http://blog.cakewalk.com/...r-x3-vst-enhancements/

OK, it says
 
Hide Related VST2 Plug-ins.
If you have both VST2 and VST3 versions of a plug-in, you can instruct SONAR to automatically hide any redundant VST2 plug-ins from SONAR’s plug-in menus.
 
I bought Sonar X3. How in the name of the gods am i expected to know that the bundled sessiondrummer is  a "redundant VST2 plug-in"?
 
 

It is not redundant. Session Drummer is a VST2 plug-in. There is no VST3 version.