The default desktop icon will launch the 32 bit version. You can get to the 64 bit version by launching through the steam library (Library>software) which will give you a popup for choosing the version.
You can also manually create a desktop shortcut for the 64 bit version - the .exe is in your steam folder:
\Steam\SteamApps\common\Cakewalk\x64\SONAR X3\SONARX3.exe