Cakewalk Sound Center is part of X3 Producer SONAR installer. Re-run the installer, select advanced install option and a pick list is presented. However, it is unusual for the installer to skip this plug-in. There is little information to go on.
The upgrade to producer was from which product?
What OS?
Which version 32bit or 64bit of SONAR is installed (or are both installed)?
After installation was the X3e update applied, see
http://www.cakewalk.com/Support/Knowledge-Base/2007013361/SONAR-X3e-Update ? Was the VST reset and rescan performed, see
http://www.cakewalk.com/Support/Knowledge-Base/2007013315/SONAR-X3-Full-VST-Rescan-Recommended-for-Upgraders ?
It is also possible if the CSC was previously installed with a 32bit host (like MC6) and an upgrade was performed to a 64bit host, the VST path in SONAR may need to be updated to include "C:\Program Data (x86)\Cakewalk\Vstplugins" in
Preferences > File > VST Settings.