I just got IK Multimedia's Modo Bass.  It's installed in Programs>IK Multimedia.  The .dll file is stored in Program Files>Cakewalk>Vst Plugins as are many of my working .dll files.  Sonar doesn't see it.  It definitely sees Cakewalk>Vst folder and I've re-scanned multiple times.  I've even created a special folder for the Modo Bass .dll, Reinstalled Modo Bass and directed it to that folder, then had Sonar scan that.  No luck.  IK Multimedia support was also out of ideas.  It worked on their Sonar Platinum, but they don't have X3 to test it on.  Any ideas would be much appreciated!
(also, just now learning about Cakewalk - ugh.)

Try a reset AND re-scan under VST preferences. Also, are you running 32 or 64-bit OS/SONAR?

Thanks Craig.  I reset and re-scanned to no avail.  I'm running 64-bit. 

Are you sure the MODO bass VST install path is included in X3's scna paths?

Yes, the .dll file is stored in Program Files>Cakewalk>Vst Plugins along with many of my working plugins.

Not 100% sure but when I installed I thought I used a default installation and that put the file here in a wind10 system:
 
c:\program files\common files\vst3\MODO BASS.vst3
 
Maybe it is still getting directed to its default?

Thanks, yes that's right.  The vst3 is in c:program files/common files.  The vst is in Program Files>Cakewalk>Vst Plugins.  I think those are both the default locations.  Either way, their both in the list of files to be scanned.

Did you try starting Sonar with "Run as Administrator"?

JonD
Did you try starting Sonar with "Run as Administrator"?

^^ THIS ^^

Special folder which you created for the Modo Bass.dll is the subfolder for Program Files>Cakewalk>Vst Plugins? Try reinstall dll in other folder, for example program files\steinberg\vstplugins or program files\vstplugins.